qa/cephfs: improve caps_helper.CapTester

qa/cephfs: improve caps_helper.CapTester

Improvement #1:

CapTester.write_test_files() not only creates the test file but also
does the following for every mount object it receives in parameters -

* carefully produces the path for the test file as per parameters
  received
* generates the unique data for each test file on a CephFS mount
* creates a data structure -- list of lists -- that holds all this
  information along with mount object itself for each mount object so
  that tests can be conducted at a later point

Untangle this mess of code by splitting this method into 3 separate
methods -

1. To produce the path for test file (as per user's need).
2. To generate the data that will be written into the test file.
3. To actually create the test file on CephFS.

Improvement #2:

Remove the internal data structure used for testing -- self.test_set --
and use separate class attributes to store all the data required for
testing instead of a tuple. This serves two purpose -

One, it makes it easy to manipulate all this data from helper methods
and during debugging session, especially while using a PDB session.

And two, make it impossible to have multiple mounts/multiple "test sets"
within same CapTester instance for the sake of simplicity. Users can
instead create two instances of CapTester instances if needed.

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