Fix empty functions causing stack failure

This patch adds a return value to several empty functions that are causing stack.sh to fail when the
Cisco Neutron plugin is enabled.

Change-Id: I43987d9cc5edc53de41c27354c3a737643d1cd43
Closes-bug: 1383273
diff --git a/lib/neutron_plugins/cisco b/lib/neutron_plugins/cisco
index 1406e37..95e0ab3 100644
--- a/lib/neutron_plugins/cisco
+++ b/lib/neutron_plugins/cisco
@@ -82,6 +82,7 @@
 }
 
 function neutron_plugin_configure_debug_command {
+    :
 }
 
 function neutron_plugin_configure_dhcp_agent {
@@ -89,6 +90,7 @@
 }
 
 function neutron_plugin_configure_l3_agent {
+    :
 }
 
 # Configure n1kv plugin
@@ -111,6 +113,7 @@
 }
 
 function neutron_plugin_configure_plugin_agent {
+    :
 }
 
 function neutron_plugin_configure_service {