deafault external_network to "public"
deafault external_network to "public"

Change-Id: I03c74f2fb47a33820e0bac4947aeaaf096feade3

yuanying/heat-kubernetes,v1k0d3n/heat-kubernetes,larsks/heat-kubernetes,rootfs/heat-kubernetes,mdshuai/heat-kubernetes,projectatomic/heat-kubernetes,jamesmarva/heat-kubernetes