Administrator password injection

Describe how to configure Compute to support admin password injection.

Closes-bug: #1254563

Backport: havana grizzly

Backport notes:

- The dashboard configuration is landing in Icehouse, it's not in
grizzly <https://review.openstack.org/56151>.

- The Libvirt configuration option is called libvirt_inject_password
in grizzly. In havana, it's "inject_password" in the [libvirt] group.

Change-Id: I63e51ad7bf45387cb79d85e937a805b3804df41a
Merge "Administrator password injection"