e2e: add backend image validation for rbd
e2e: add backend image validation for rbd

validate backend rbd images count in each
E2E test cases. This helps a lot to catch
the issues in each test case.

Signed-off-by: Madhu Rajanna <60fbf37a92c2a0908e31296d18dadbec5a4dae17@gmail.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi