qa/cephfs: fix caps_helper.py

qa/cephfs: fix caps_helper.py

Replace calls to run_cluster_cmd() by calls to run_ceph_cmd().

Ideally, a backport should have fixed this issue but the method under
inspection here, "get_mon_cap_from_keyring()", has gone through lots of
changes, compared to the main branch, along with rest of caps_helper.py.
To avoid backporting large amount of QA changes, writing a specific fix
will be easier and faster.

This will also help with rest of Quincy backports since this patch
series/PR makes an important change to QA code.

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