Add assertNotEmpty to avoid IndexError
Add assertNotEmpty to avoid IndexError

In VolumePoolsAdminTestsJSON, if no cinder pools is listed,
we get "IndexError: list index out of range", so we should
check whether cinder_pools is empty before using it.

Change-Id: I55c163993b4956458b8658dae9d11b61171b8b55
Closes-Bug: #1844772

cisco-openstack/tempest,openstack/tempest,openstack/tempest,cisco-openstack/tempest