Allow dhcp_server to be set from new field

Now that dhcp_server and gateway can be different, update dnsmasq
to hand out the gateway if we have a single ip address and they
don't match.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: Id040ce1fb0f889418815568ff6c80402b69bf09e
Move libvirt RBD utilities to a new file

This will make it easier to share rbd-related code with cinder and glance.
Port the applicable unit tests over from cinder.

Closes-Bug: #1346092
Co-Authored-By: Dmitry Borodaenko <dborodaenko@mirantis.com>
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Dmitry Borodaenko <dborodaenko@mirantis.com>
Change-Id: I614b8e2bf0ea9e71a73772cbabcf812b39fa8a34
Merge "Allow dhcp_server to be set from new field"
Merge "Move libvirt RBD utilities to a new file"