Cherrypick RBD commits

This PR brings in Josh's rbd changes into our havana branch, addresses DE727 - Allows us to boot ephemeral instances on compute nodes with insufficient local storage by utilizing 'libvirt_images_type = rbd' . This, along with the appropriate inclusion of ceph configurations files/keyring achieves the same functionality as the old 'boot volume patch' we reverted.

Closes-rally-bug: DE727
Closes-rally-bug: DE642
Closes-bug: 1226351