main: confirm cleanup at first signal received

website: bump Ubuntu example from 12.04.2 to 12.04.3

Ubuntu has removed the 12.04.2 release ISOs from their mirrors when they released 12.04.3. This impacted the URL and the Checksum contained in the example.json which returned an HTTP 404 from Ubuntu's Mirrors.

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`