Add post-test hook for testing evacuate

This adds a post-test bash script to test evacuate
in a multinode job.

This performs two tests:

1. A negative test where we inject a fault by stopping
   libvirt prior to the evacuation and wait for the
   server to go to ERROR status.

2. A positive where we restart libvirt, wait for the
   compute service to be enabled and then evacuate
   the server and wait for it to be ACTIVE.

For now we hack this into the nova-live-migration
job, but it should probably live in a different job
long-term.

Change-Id: I9b7c9ad6b0ab167ba4583681efbbce4b18941178

playbooks/legacy/nova-live-migration/run.yaml

@@ -34,6 +34,7 @@
           export DEVSTACK_GATE_TOPOLOGY="multinode"
           function post_test_hook {
               /opt/stack/new/nova/nova/tests/live_migration/hooks/run_tests.sh
+              $BASE/new/nova/gate/test_evacuate.sh
           }
           export -f post_test_hook
           cp devstack-gate/devstack-vm-gate-wrap.sh ./safe-devstack-vm-gate-wrap.sh