test_rbd.py: refactor cloning tests

test_rbd.py: refactor cloning tests

Move into a separate class that requires layering to be enabled,
so the common step of creating and deleting a clone doesn't
need to be repeated in each test.

Move flatten tests into a subclass so they can be run separately
more easily.

Move the checks for the layering feature into a generic decorator
that skips tests if the specified feature is not being used.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>