Tests for volume<->image dependencies

Detect problems related to Cinder volumes not allowing
Glance images to be deleted.

This adds the option
CONF.volume_feature_enabled.volume_image_dep_tests.
This defaults to True because it is expected to pass on all
configurations other than pre-Caracal RBD backends.

Change-Id: I5fee23951958fc0031b59ce437a963c4cea28529
diff --git a/cinder_tempest_plugin/config.py b/cinder_tempest_plugin/config.py
index 78dd6ea..53222b8 100644
--- a/cinder_tempest_plugin/config.py
+++ b/cinder_tempest_plugin/config.py
@@ -22,6 +22,9 @@
     cfg.BoolOpt('volume_revert',
                 default=False,
                 help='Enable to run Cinder volume revert tests'),
+    cfg.BoolOpt('volume_image_dep_tests',
+                default=True,
+                help='Run tests for dependencies between images and volumes')
 ]
 
 # The barbican service is discovered by config_tempest [1], and will appear