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/trove b/lib/trove
index 9c91024..1fd011a 100644
--- a/lib/trove
+++ b/lib/trove
@@ -38,6 +38,10 @@
     TROVE_BIN_DIR=$(get_python_exec_prefix)
 fi
 
+# Tell Tempest this project is present
+TEMPEST_SERVICES+=,trove
+
+
 # Functions
 # ---------