Split base snapshots_client into v1 and v2

This patch splits the base client into v1 and v2 following the model
from the patch at [1] and also migrates those clients to tempest.lib.

[1] https://review.openstack.org/#/c/359442/

Change-Id: I03c254210aeff80173ced68cfa3f3e36ede10d22
7 files changed