Address certain mobile issues

Explain how packer works with atlas

Clarify the plugin discovery search & priority orders.

Corrected invalid boot command section for Parallels, Virtualbox, and VMware ISO builders

Better error reporting when a config key in template is Unknown

This patch will allow to fix the following bug much faster:

```
1 error(s) occurred:

* Unknown configuration key: output_directory
```

Related configuration:

```
"output_directory ": "build/sl_base/",
```

After the patch, the error reporting will be:

```
1 error(s) occurred:

* Unknown configuration key: "output_directory¤"
```

Address go vet reports

Fixes the following vet reports:

provisioner/salt-masterless/provisioner.go:119: arg cmd for printf verb %s of wrong type: *github.com/mitchellh/packer/packer.RemoteCmd
provisioner/salt-masterless/provisioner.go:121: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:142: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:157: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:172: arg err for printf verb %d of wrong type: error
provisioner/salt-masterless/provisioner.go:216: arg err for printf verb %d of wrong type: error

Fixes the following vet report:

provisioner/puppet-masterless/provisioner.go:196: missing argument for Errorf(%d): format reads arg 1, have only 0 args

Fixes the following vet reports:

post-processor/vagrant-cloud/client.go:81: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:99: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:131: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:156: arg resp for printf verb %s of wrong type: *net/http.Response
post-processor/vagrant-cloud/client.go:175: arg resp for printf verb %s of wrong type: *net/http.Response

Fixes the following vet report:

post-processor/vagrant/post-processor_test.go:135: possible formatting directive in Fatal call

Fixes the following vet reports:

packer/template_test.go:496: possible formatting directive in Fatal call
packer/template_test.go:528: possible formatting directive in Fatal call

Fixes the following vet report:

communicator/ssh/communicator_test.go:88: range variable newChannel captured by func literal

Fixes the following vet reports:

common/step_create_floppy_test.go:79: possible formatting directive in Fatal call
common/step_create_floppy_test.go:89: possible formatting directive in Fatal call
common/step_create_floppy_test.go:180: possible formatting directive in Fatal call
common/step_create_floppy_test.go:190: possible formatting directive in Fatal call

Fixes the following vet report:

builder/vmware/iso/builder_test.go:178: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/vmware/common/step_shutdown_test.go:130: missing argument for Fatalf("%s"): format reads arg 1, have only 0 args

Fixes the following vet report:

builder/virtualbox/iso/builder_test.go:86: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/qemu/builder_test.go:163: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet report:

builder/parallels/iso/builder_test.go:78: arg b.config.DiskSize for printf verb %s of wrong type: uint

Fixes the following vet reports:

builder/openstack/artifact.go:44: arg a.ImageId for printf verb %d of wrong type: string
builder/openstack/step_wait_for_rackconnect.go:24: arg server for printf verb %s of wrong type: *github.com/mitchellh/gophercloud-fork-40444fb.Server

Fixes the following vet reports:

builder/googlecompute/step_create_instance_test.go:42: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:29: possible formatting directive in Fatal call
builder/googlecompute/step_teardown_instance_test.go:39: possible formatting directive in Fatal call

Fixes the following vet reports:

builder/digitalocean/builder_test.go:267: arg b.config.SSHUsername for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:300: arg b.config.RawSSHTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:341: arg b.config.RawStateTimeout for printf verb %d of wrong type: string
builder/digitalocean/builder_test.go:382: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool
builder/digitalocean/builder_test.go:397: arg b.config.PrivateNetworking for printf verb %s of wrong type: bool

Fixes the following vet report:

builder/amazon/ebs/step_create_ami.go:90: arg resp.Return for printf verb %s of wrong type: bool

Fixes the following vet reports:

builder/amazon/common/step_ami_region_copy.go:81: arg target for printf verb %s of wrong type: github.com/mitchellh/goamz/aws.Region
builder/amazon/common/step_ami_region_copy.go:93: arg target for printf verb %s of wrong type: github.com/mitchellh/goamz/aws.Region

docs/templates: add note about escaping

post-processor/vagrant: fix test failure on Windows

Add Rackconnect support to openstack builder