qa/cephfs: CephFSTestCase.create_client() must keyring

qa/cephfs: CephFSTestCase.create_client() must keyring

Replace call to run_ceph_cmd() by call to get_ceph_cmd_stdout() in
method qa.tasks.cephfs.cephfs_test_case.CephFSTestCase.create_client().
run_ceph_cmd() will not return keyring which is wrong.
get_ceph_cmd_stdout() will return the stdout of "ceph auth add"
command, which is the keyring that is expected to be returned by
CephFSTestCase.create_client().

Fixes: https://tracker.ceph.com/issues/62246
Signed-off-by: Rishabh Dave <ridave@redhat.com>