Generate Tempest service list rather than hard-code it

The list of services that Tempest used to set its 'service_available'
config values was hard-coded. To be plugin-friendly have each
service (project) add its name to the TEMPEST_SERVICES variable
and use that for setting the 'service_avilable' values.

Change-Id: I208efd7fd0798b18ac2e6353ee70b773e84a2683
diff --git a/lib/nova b/lib/nova
index dbaa3f5..9db19ed 100644
--- a/lib/nova
+++ b/lib/nova
@@ -122,6 +122,9 @@
 TEST_FLOATING_POOL=${TEST_FLOATING_POOL:-test}
 TEST_FLOATING_RANGE=${TEST_FLOATING_RANGE:-192.168.253.0/29}
 
+# Tell Tempest this project is present
+TEMPEST_SERVICES+=,nova
+
 
 # Functions
 # ---------