qa/cephfs: make it easier to use xfstests-dev using vstart_runner

qa/cephfs: make it easier to use xfstests-dev using vstart_runner

The intention of this re-arrangement is to make it easier for developers
to use an already cloned and built xfstests-dev repo  with
vstart_runner.py.

The re-arrangements are -

* move tearDown() closer to setUp() (so that devs can
  enable/disable parts of them easily),

* carve out separate methods (so that these codeblocks can be disabled
  in a single go) for -
  ** deleting groups and users,
  ** deleting xfstests-dev and xfsprogs-dev repo and
  ** building xfstests-dev repo

* carve out method for deleting xfstests-dev and xfsprogs-dev repos

* add self.xfstests_repo_path comment (for devs to set path to
  xfstests-dev repo), and,

* add a note showing how to use already cloned and built xfstests-dev
  repo

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