Enrich e2e test to flatten-rbd-pvc command

Related discussion

flatten-rbd-pvc comand only has one e2e test case. It only checks the PVC-PVC clone case. We should check other cases . The additional test cases would be as follows:

PVC restore: pass without deleting the temporary image corresponding to VolumeSnapshot.
static PVC: fail
in-use PVC without allow-in-use option: fail
in-use PVC with allow-in-use option: pass
non-cloned/restored PVC: fail
not-bounded PVC: fail

rook / kubectl-rook-ceph

Enrich e2e test to flatten-rbd-pvc command #294