qa/cephfs: add and use run_ceph_cmd()

qa/cephfs: add and use run_ceph_cmd()

Instead of writing something as long as
"self.mds_cluster.mon_manager.run_cluster_cmd()" to execute a command,
let's add a helper method to class CephFSTestCase and use it instead.

With this, running a command becomes simple - "self.run_ceph_cmd()".

Signed-off-by: Rishabh Dave <ridave@redhat.com>