Add description for different interfaces

when run kolla-ansible command, the priority of globals.yml is
higher than multinode, we should comment out the vars in globals.yml
and make mutinode's vars effect.

Change-Id: I0fe389ac1e5155f9779284ecc5afb524743faf16

ansible/inventory/multinode

@@ -22,7 +22,8 @@ compute01
 monitoring01
 
 # When compute nodes and control nodes use different interfaces,
-# you can specify "api_interface" and other interfaces like below:
+# you need to comment out "api_interface" and other interfaces from the globals.yml
+# and specify like below:
 #compute01 neutron_external_interface=eth0 api_interface=em1 storage_interface=em1 tunnel_interface=em1
 
 [storage]