Expand cluster-template usage validation message
Expand cluster-template usage validation message

With the introduction of a CLI, users are presented directly with the
error messages sent from the API service. Specifically, when cluster
templates are delete a usage check validates that there are no users
of the template. This commit expands the message returned from the
check to include clusters where the template is in use.

Change-Id: I516d107edc93e24832ccb952f3e68defba92adcd

crobby/sahara,ekasitk/sahara,ekasitk/sahara,bigfootproject/sahara,tellesnobrega/storm_plugin,esikachev/scenario,matips/iosr-2015,citrix-openstack-build/sahara,keedio/sahara,redhat-openstack/sahara,zhujzhuo/Sahara,henaras/sahara,openstack/sahara,keedio/sahara,zhangjunli177/sahara,crobby/sahara,xme1226/sahara,henaras/sahara,egafford/sahara,xme1226/sahara,bigfootproject/sahara,citrix-openstack-build/sahara,tellesnobrega/sahara,keedio/sahara,henaras/sahara,crobby/sahara,zhangjunli177/sahara,mapr/sahara,esikachev/sahara-backup,esikachev/sahara-backup,redhat-openstack/sahara,tellesnobrega/storm_plugin,mapr/sahara,xme1226/sahara,bigfootproject/sahara,mapr/sahara,esikachev/sahara-backup,egafford/sahara,zhujzhuo/Sahara,openstack/sahara,citrix-openstack-build/sahara,tellesnobrega/storm_plugin,matips/iosr-2015,zhujzhuo/Sahara,esikachev/scenario,ekasitk/sahara,tellesnobrega/sahara,zhangjunli177/sahara,matips/iosr-2015,esikachev/scenario,redhat-openstack/sahara