Improve snapshot data integrity test

Currently the test require lot of resources i.e. 4 instance.
This patch uses recommendation from the comments of initial patch[1]
i.e. creates 1 instance, attach volumes to it, snapshot etc

[1] https://review.opendev.org/c/openstack/cinder-tempest-plugin/+/702495

Depends-On: https://review.opendev.org/c/openstack/barbican/+/796059

Change-Id: I151b034dd28834290ffeb3ce9c052a6c3bbfb7d8
1 file changed