qa/cephfs: upgrade caps_helper.CapTester.run_mon_cap_tests()

qa/cephfs: upgrade caps_helper.CapTester.run_mon_cap_tests()

With this commit, the design of run_mon_cap_tests() is now aligned with
rest of CapTester -- it's not meant to be inherited by test class
anymore and it is to be called by using CapTester instance.

This commit also changes working of this method. Now instead of
obtaining FS names from Python objects representing FSs, it obtains
those names from MON cap. This removes the need to pass the FS objects
around.

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