modifies the test to sort based on UUID

This patch modifies the volume-list filter test to perform the sorting
based on UUID instead of created_at timestamp as this timestamp might
be same for two volumes but UUID for every volume is unique.

Change-Id: Ibe1ab23c12ad8afe8d5adab6fe0c617445adc90d
Closes-Bug: 1313970
Merge "modifies the test to sort based on UUID"
Merge branch 'master' of https://github.com/openstack/tempest