qa/ceph_manager: accepts commands as str too

qa/ceph_manager: accepts commands as str too

Modify CephManager.run_cluster_cmd() to accept command arguments as
string as well since typing commands as strings is much lesser effort
than typing as list. This brings the interface a step closer to
teuthology.orchestra.remote.run()'s interface since it too can accept
commands arguments as string.

The change in cephfs_test_case.py is just to allow testing this PR
locally and on teuthology.

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