Add nova config option to service_available group

This commit adds a new config option, nova, to the service_available
config group. This option is used to specify whether nova is running
or not. It also add a skip to base compute api test class.

Change-Id: I658f70f259734e366cecce1c8e0409255351af87
diff --git a/tempest/config.py b/tempest/config.py
index 83ea25c..5eb52dc 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -551,6 +551,9 @@
     cfg.BoolOpt('swift',
                 default=True,
                 help="Whether or not swift is expected to be available"),
+    cfg.BoolOpt('nova',
+                default=True,
+                help="Whether or not nova is expected to be available"),
 ]