Don't create keystone account for disabled service

If service is disabled, don't expect to create the corresponding
keystone account.

Change-Id: I007088862de1c8643eca3a6b9b313f0125b9b8d1
diff --git a/stack.sh b/stack.sh
index 4cee385..f08d56f 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1057,11 +1057,18 @@
     fi
 
     create_keystone_accounts
-    create_nova_accounts
-    create_glance_accounts
-    create_cinder_accounts
-    create_neutron_accounts
-
+    if is_service_enabled nova; then
+        create_nova_accounts
+    fi
+    if is_service_enabled glance; then
+        create_glance_accounts
+    fi
+    if is_service_enabled cinder; then
+        create_cinder_accounts
+    fi
+    if is_service_enabled neutron; then
+        create_neutron_accounts
+    fi
     if is_service_enabled swift; then
         create_swift_accounts
     fi