put the stackforge library installation behind a conditional

we don't actually want to drag in stackforge libraries all the
time, instead ensure that we have enabled stackforge_libs before
doing it.

Change-Id: Ic1c2e3d19e106a2aa0db9725d16a8b207546c23d
diff --git a/stack.sh b/stack.sh
index 45604da..6de7599 100755
--- a/stack.sh
+++ b/stack.sh
@@ -631,7 +631,9 @@
 install_oslo
 
 # Install stackforge libraries for testing
-install_stackforge
+if is_service_enabled stackforge_libs; then
+    install_stackforge
+fi
 
 # Install clients libraries
 install_keystoneclient