qa/cephfs: add and use get_ceph_cmd_result()

qa/cephfs: add and use get_ceph_cmd_result()

To run a command and get its return value, instead of typing something
as long as "self.mds_cluster.mon_manager.raw_cluster_cmd_result" add a
hepler method in CephFSTestCase and use it. This makes this task very
simple - "self.get_ceph_cmd_result()".

Also, remove method CephFSTestCase.run_cluster_cmd_result() in favour of
this new method.

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