qa/cephfs: when cmd output is not needed call run_ceph_cmd()

qa/cephfs: when cmd output is not needed call run_ceph_cmd()

instead of get_ceph_cmd_stdout().

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit c7c38ba558e089c51d71e0b0713b3e0a368b9699)

Conflicts:
	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.
	- The line located right before one of the patches in this
	  commit was modified in latest Reef branch, thus creating
	  conflict when PR branch was rebased on latest Reef.