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/savanna b/lib/savanna
index c7d59f7..176f290 100644
--- a/lib/savanna
+++ b/lib/savanna
@@ -40,6 +40,10 @@
     SAVANNA_BIN_DIR=$(get_python_exec_prefix)
 fi
 
+# Tell Tempest this project is present
+TEMPEST_SERVICES+=,savanna
+
+
 # Functions
 # ---------