in e2e/volumes.go: give time to allow pod cleanup and volume unmount happen before volume server exit;
skip cinder volume test if not running with openstack provider

comment on why pause before containerized server is stopped in volume e2e tests, fix #24100

updates NFS server image to 0.6, per #22529

fix persistent_volume e2e test: test cleanup doesn't expect client pod; delete PV after test
Signed-off-by: Huamin Chen <hchen@redhat.com>