Allow roles who don't want a config file to skip passing one in

This worked before because edx_service_config was undefined, now
we have a default empty dict which causes the same behavior (skips the
app file generation step).  This is used by xqwatcher and harstorage.

If we prefer to change those roles instead, I'm not wedded to this fix.

playbooks/roles/edx_service/defaults/main.yml

@@ -17,6 +17,12 @@
 edx_service_name: edx_service
 
 edx_service_repos: []
+
+# A few roles meta this role but don't need a config file written
+# this allows them to not pass a config and the tasks will skip
+# and not write out a config at all.
+edx_service_config: {}
+
 #
 # OS packages
 #