builder/digitalocean: send a "shutdown" before snapshotting

This change adds base support for an openstack builder.
Thank you to Rackspace for providing cloud assets to complete this work
and @sam-falvo for working with us on the perigee/gophercloud changes.

Chef-solo provisioner first pass.

Merge branch 'provisioner-chef-solo' of https://github.com/jvandyke/packer into jvandyke-provisioner-chef-solo

Conflicts:
	config.go

website: Actual md5sum as Ubuntu Hashesh webpage is out of date.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

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.