qa/cephfs: add and use get_ceph_cmd_stdout()

qa/cephfs: add and use get_ceph_cmd_stdout()

Add method get_ceph_cmd_stdout() to class CephFSTestCase so that one
doesn't have to type something as long as
"self.mds_cluster.mon_manager.raw_cluster_cmd()" to execute a
command and get its output. And delete and replace
CephFSTestCase.run_cluster_cmd() too.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 13168834e374120d9c319a120a35768ddbc70de5)

Conflicts:
	qa/tasks/cephfs/caps_helper.py
	- This file is very different in Reef.
	qa/tasks/cephfs/test_mirroring.py
	- Commit e4dd0e41a3a0 was not present on main but it is now
	  present on main as well as on Reef, which leads to conflict.
	- On Reef branch, the line before that patch in this commit was
	  thus creating a conflict when the PR branch for this commit
	  series was rebased on latest Reef.