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>
(cherry picked from commit f8f2154e54e202996b24904162da6022081e9d93)

Conflicts:
	qa/tasks/cephfs/test_damage.py
	This file is slightly different because this commit
	c8f8324ee2fae48e8d3c2bbdbf45cc9ffe46fd4c was merged on main and
	backported after the commit being cherry-picked here was merged
	in main.