builder/amazon/common: fix check for vpcid

check for VpcId was incorrectly checking if VPC is empty, instead of not empty.

/cc @jasonberanek - Just adding this as well because I see this being
common as well.

common: Fix URL and Relative Windows file path support #235/#239

/cc @rgarcia Should just pass it in as a param

/cc @rgarcia

/cc @rgarcia - "Message" in Ui is the non-bold colored messages and
are meant for sub-messages or steps part of a larger message. "Say"
is used for the bold stuff. This is more in line wit how Packer works.

/cc @rgarcia - Not sure why this was capitalized to begin with

/cc @rgarcia - Since you opened the pull request, common config
decoding stuff has been pulled out into the common Package. This
simplifies a lot of code.

/cc @rgarcia - This just forces the tests to make sure the package
even builds. Better than nothing.

/cc @rgarcia - I don't know of any Go packages that follow this naming
convention so I just lowercased it.

provisioner/salt-masterless: masterless salt provisioner

We shouldn't need a different one for each driver ...

Conflicts:
	builder/vmware/driver_workstation9.go

We have two choices for VMware on Linux.  We already support
Workstation, add support for Player too.

VMware Player doesn't come with vmware-vdiskmanager, so fall back to
using qemu-img if we can find that.

Conflicts:
	builder/vmware/driver.go

builder/amazon/common: handle missing source AMI error