common: do not regenerate a cluster fsid if cluster exists
This commit solves the situation where you lost your fetch directory and
you are running ansible against an existing cluster. Since no fetch
directory is present the file containing the fsid doesn't exist so we
are creating a new one. Later the ceph.conf gets updated with a wrong
fsid which causes problems for clients and ceph processes.
Closes: #1148 Signed-off-by: Sébastien Han <seb@redhat.com>