Isolate creating service users

The code for creating service users is almost exactly the same. Abstract
this into a function that can be reused and standardized.

Change-Id: I3a4edbff0a928da7ef9b0097a5a8d508fdfab7ff
diff --git a/lib/glance b/lib/glance
index bee57a3..0340c21 100644
--- a/lib/glance
+++ b/lib/glance
@@ -232,8 +232,7 @@
 function create_glance_accounts {
     if is_service_enabled g-api; then
 
-        local glance_user=$(get_or_create_user "glance" "$SERVICE_PASSWORD")
-        get_or_add_user_role service $glance_user $SERVICE_TENANT_NAME
+        create_service_user "glance"
 
         # required for swift access
         if is_service_enabled s-proxy; then