website: Actual md5sum as Ubuntu Hashesh webpage is out of date.

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