replace deprecated libvirt_* options for rbd

All libvirt_* options for Nova were deprecated in Icehouse and removed
in Juno, this commit replaces the rbd related deprecated options with
corresponding options in the [libvirt] section of nova.conf.

Change-Id: If9670e6df8dccbbb659d48d59e6a60b5583d68ac
Partial-Bug: #1386735
Merge "replace deprecated libvirt_* options for rbd"