Additional changes necessary for Nimble deployment

This is a collection of changes that needs to be integrated for Nimble
SAN deployment. It is a reworking of the original code to handle various
aspects of doing this live. These include:

- update to ansible.cfg to point at our roles directory explicitly
- add forwarding agent capability to ursula
- a deploy-nimble-cinder playbook
- updates to sensu checks for nimble
- and ability to push private keys to a stack
Add ssh_private_keys default to common

Without an empty default the run will fail if ssh_private_keys is not
defined.
Merge pull request #512 from blueboxgroup/nimble3

Additional changes necessary for Nimble deployment