librbd: Python unit tests now use unique pools and images

RBD python unit tests no longer utilize the 'rbd' pool for
test cases.  Instead, a new temporary pool is created and
deleted. Additionally, each unit test now uses a unique
image name to reduce the possibility of test case failures
affecting subsequent tests.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2898 from ceph/wip-rbd-python-tests

librbd: Python unit tests now use unique pools and images

Reviewed-by: Josh Durgin <jdurgin@redhat.com>