support for managing ssh keys

This change permits provisioning ssh public and private keys as part of
creating the "nova" user.  There are no changes to the default behavior
of the module.  This changes introduces three new parameters:

- nova_public_key
- nova_private_key
- nova_shell

Change-Id: Ib61f4574843a7e5ada9a08692f4488cb844c9eee
Implements: blueprint provision-ssh-keys