packer/plugin: confirm cleanup at first signal received
Bump Ubuntu 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.
Merge pull request #342 from masteinhauser/patch-1

website: bump Ubuntu example from 12.04.2 to 12.04.3
Merge pull request #338 from mitchellh/signal-confirm

main: confirm cleanup at first signal received
Update CHANGELOG
Actual md5sum as Ubuntu Hashesh webpage is out of date.
Merge pull request #343 from masteinhauser/patch-1

website: Actual md5sum as Ubuntu Hashesh webpage is out of date.