This patch removes the nova_libvirt_secret container volume because
it is a complete antipattern, and during testing, I found that
it causes problems. When it was necessary to copy libvirt secrets
from /etc/kolla/nova-libvirt/secrets, the container logs reported that
the resource is busy - precisely because it was a mounted container
volume. This, of course, is unnecessary because the secrets are copied
to the kolla host in /etc/kolla/nova-libvirt/secrets.

Closes-Bug: #2073678
Change-Id: I715a6a95f9d32d62a8199727ddbaddd0dd7baa2d