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/etc/tempest.conf.sample b/etc/tempest.conf.sample
index 32fe484..3579026 100644
--- a/etc/tempest.conf.sample
+++ b/etc/tempest.conf.sample
@@ -356,3 +356,5 @@
 glance = True
 # Whether or not swift is expected to be available
 swift = True
+# Whether or not nova is expected to be available
+nova = True