Add a config option to select compute xml api

This commit adds a config option to select whether the nova v2 api
has xml enabled.

Change-Id: I5db2b782e71d99705c9f071a14ab850afbeddeb8
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index a3295eb..343a39a 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -37,6 +37,9 @@
     def setUpClass(cls):
         cls.set_network_resources()
         super(BaseComputeTest, cls).setUpClass()
+        if getattr(cls, '_interface', None) == 'xml' and cls._api_version == 2:
+            if not CONF.compute_feature_enabled.xml_api_v2:
+                raise cls.skipException('XML API is not enabled')
 
         # TODO(andreaf) WE should care also for the alt_manager here
         # but only once client lazy load in the manager is done