qa/cephfs: upgrade and rename class CapsHelper

qa/cephfs: upgrade and rename class CapsHelper

Modify caps_helper.py such that calling code does't have to store
returned values just to pass those values again to a method in
caps_helper.py. This is a common pattern where write_test_files()
return 3 values and all 3 passed as it is to run_cap_tests().

The easy way to do it is to upgrade caps_helper so that it can be used
an object and not just as a class supplying helper methods. The return
values will be stored by the object internally and thus resued. In case
of testing multiple FSs inside a single test method, we'll now need
multiple instances of this class to keep those values separate.

And since CapsHelper is not just a class supplying helper methods
anymore, it's being renamed to CapTester.

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