Ceph: undefine secret from nova.conf when deleting secret

At teardown time we delete the libvirt secret but leave
nova.conf referencing the stale UUID.  Remove it to make
debugging and repeated runs less error-prone.

Additionally, only try to delete secret if it Cinder is
enabled (causing it to be defined when deploying devstack).

Change-Id: Id5e1290b8dcfd79238f02e31084ab12c3d6aed5f
Merge "Ceph: undefine secret from nova.conf when deleting secret"