check for rabbitmq host to prevent headache when its not provided

Just a simple check in the client_service recipe.  If you are deploying a client and do not have a rabbitmq host the service fails to start.  This fails your chef run and the errors bubbled back up to chef aren't helpful for debugging the issue.

This will enforce providing a rabbitmq host and clearly let you know when you do not so you don't chase your tail figuring out why your service isn't starting.