Fixes for hacking 0.6

- Add missing License Header to exc.
- Use python 3 compatible print.

Change-Id: Ibbcbfadffb978f9d8789e1fbc0c0819da06489d3
Run forever if a command is configured

This makes os-collect-config stay resident and prepares it for a more
event based operation when the Heat API is ready for that via longpoll
or callbacks or something else.

Change-Id: Ic91f2201d504e9f8e0ada6d34a7d6d94785aec87
Use heat-cfntools path hint for cfn.metadata_url

Heat passes this in via cloud-init to assist heat-cfntools in finding the
cfn metadata server url. This will likely be the primary location used,
and cfn.metadata_url will be an override.

Also doing some cleanup, removed unused cfn.EC2_METADATA_URL and
reordered and regrouped imports.

Change-Id: I52093bdd8a4b939ae06cf483535ed5d5a0e273d4