Improved Cinder snapshot tests

In some NFS based drivers, including the base
NFS driver[1] there's 2 flows in the snapshoting operation:
online and offline. For online snapshots, Cinder calls Nova
that cames to action and use libvirt to create and manage image
snapshot files. For offline operations, Cinder alone uses qemu-img
to deal with snapshots. This test adds several combination between
online/offline snapshoting, making sure that snapshots created
online is compatible with offline and vice-versa.

[1] https://review.openstack.org/#/c/147186/

Change-Id: I34be2f311adddbb8faa3dace9e8b6bc05b4d93f2
2 files changed