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/neutron b/lib/neutron
index 960f11b..68dfd4a 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -237,6 +237,10 @@
     Q_USE_SECGROUP=False
 fi
 
+# Tell Tempest this project is present
+TEMPEST_SERVICES+=,neutron
+
+
 # Functions
 # ---------