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/glance b/lib/glance
index 55d5fb3..2d41ea4 100644
--- a/lib/glance
+++ b/lib/glance
@@ -52,6 +52,9 @@
 # Glance connection info.  Note the port must be specified.
 GLANCE_HOSTPORT=${GLANCE_HOSTPORT:-$SERVICE_HOST:9292}
 
+# Tell Tempest this project is present
+TEMPEST_SERVICES+=,glance
+
 
 # Functions
 # ---------