Instead of pulling the plug on a droplet with the "poweroff" command,
we first send a soft shutdown to the API, then we poweroff to allow
the snapshot to properly complete.

Sending just a shutdown and then snapshotting wasn't as reliable as
sending the poweroff manually, for reasons unknown to me.

This fixes #332.

Conflicts:
	provisioner/salt-masterless/provisioner.go
	website/source/docs/provisioners/salt.html.markdown

post-processor/vagrant: proper output ConfigTemplate validation [GH-324]

post-processor/vagrant: support for Virtualbox OVA artifacts

This improves reboot handling robustness

-apply same output validation steps found in builder specific Configure
functions

/cc @jmassara

/cc @jmassara

/cc @jmassara - This has to go before the modify attributes so that
the attributes are properly modified on all resulting AMIs.

/cc @jmassara

amazon/common: Added AMI CopyImage support

builder/vmware: Enable customization of VMDK type with `disk_type_id`