qa/cephfs: simplify some code in test_multifs_auth.py

qa/cephfs: simplify some code in test_multifs_auth.py

test_mutlifs_auth.TestMDSCaps._create_client() not only creates a client
but also generate caps strings for the client as per the parameter this
method receives and and then writes the keyring to all remote machines.
This creates confusion when reading code on test methods in TestMDSCaps.
Let's re-arrange this code such that this confusion is avoided.

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