Install agent packages when running agents like DHCP or L3

This is because these agents rely on quantum agent packages as
much as the L2 agent.

Fixes bug #1190050

Change-Id: I8b0c5667f86b22e3727a316f7bb0500d0d0ba637
diff --git a/stack.sh b/stack.sh
index 24a3d5f..4089531 100755
--- a/stack.sh
+++ b/stack.sh
@@ -584,7 +584,7 @@
     install_database
 fi
 
-if is_service_enabled q-agt; then
+if is_service_enabled quantum; then
     install_quantum_agent_packages
 fi