qa/cephfs: add helper methods to filesystem.py

qa/cephfs: add helper methods to filesystem.py

Add run_ceph_cmd(), get_ceph_cmd_stdout() and get_ceph_cmd_result() to
class Filesystem so that running Ceph command is easier. This affects
not only methods inside class Filesystem but also methods elsewhere that
uses instance of class Filesystem to run Ceph commands.

Instead of "self.fs.mon_manager.raw_cluster_cmd()" writing
"self.fs.run_ceph_cmd()" will suffice.

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