Don't define virsh secret if already defined

We test to see if the ceph secret already has been defined before
installing the secret again.

This is mainly done to keep our Foreman runs clean, as it reports
on hosts that have had changes applied. Without the 'unless' clause
any node that has the nova compute class installed will report as
having changes applied.

Change-Id: I05e83f07570d4a0e67a700bd9d9030eef1c261c3
(cherry picked from commit 69b7a3c36645630aa668fe60639417fcbc24ce32)