Add volume-backed evacuate test
Add volume-backed evacuate test

This adds a volume-backed instance evacuate scenario
to the test_evacuate post-test script.

Change-Id: I37120d9ce02de6dadbd279de195d2f289c891123

rahulunair/nova,mikalstill/nova,klmitch/nova,rahulunair/nova,klmitch/nova,mikalstill/nova,klmitch/nova,mikalstill/nova,rahulunair/nova,openstack/nova,mahak/nova,mahak/nova,klmitch/nova,openstack/nova,mahak/nova,openstack/nova