Add CONF.compute_feature_enabled.volume_backed_live_migration

The test_volume_backed_live_migration test has been skipped for some
time because of a bug [1] which could possibly be resolved with newer
libvirt and qemu package versions available in the pike UCA.

This changes the unconditional skip to a config option controlled skip
so that we can try re-enabling this test on the master branch for nova
changes.

[1] https://bugs.launchpad.net/nova/+bug/1524898

Change-Id: I78ceb1aaa3e96f6b76fa94aba50527022c3e2d45

releasenotes/notes/volume-backed-live-mig-5a38b496ba1ec093.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    A new boolean configuration option
+    ``[compute-feature-enabled]/volume_backed_live_migration`` has been added.
+    If enabled, tests which validate the behavior of Nova's *volume-backed live
+    migration* feature will be executed. The option defaults to ``False``.

tempest/api/compute/admin/test_live_migration.py

@@ -132,7 +132,9 @@ class LiveMigrationTest(base.BaseV2ComputeAdminTest):
     def test_live_block_migration_paused(self):
         self._test_live_migration(state='PAUSED')
 
-    @decorators.skip_because(bug="1524898")
+    @testtools.skipUnless(CONF.compute_feature_enabled.
+                          volume_backed_live_migration,
+                          'Volume-backed live migration not available')
     @decorators.idempotent_id('5071cf17-3004-4257-ae61-73a84e28badd')
     @utils.services('volume')
     def test_volume_backed_live_migration(self):

tempest/config.py

@@ -473,6 +473,10 @@ ComputeFeaturesGroup = [
                 default=False,
                 help='Does the test environment support in-place swapping of '
                      'volumes attached to a server instance?'),
+    cfg.BoolOpt('volume_backed_live_migration',
+                default=False,
+                help='Does the test environment support volume-backed live '
+                     'migration?'),
 ]