qa/cephfs/cap_tester: simplify CapTester and its instantiation

qa/cephfs/cap_tester: simplify CapTester and its instantiation

Class CapTester contains two distinct immiscible group of methods: one
that tests MON caps and other that tests MDS caps. When using CapTester
for the former reason the instantiation neither needs mount object and
the path where files for testing will be created nor it needs to run the
method that creates files for testing rw permissions. When using
this class for latter the case is the exact opposite.

Create 2 separate classes for each of these purpose and class that
inherits both of these classes so that instantiating the class becomes
as simple as it can be.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit f0ffade0525674b1f851797b1107060d8aa93d5c)

Conflicts:
 - qa/tasks/cephfs/caps_helper.py:
   This file went through changes due to merging of commit 59c9104c54d6.