app/app: created UnsetCName method

This method calls the provisioner.UnsetCName and sets the app.CName
attribute to "" and saves in the database.
router/hipache: changing SetCName to remove old CName record from redis

This allows a user to run the set-cname command multiple times, without
have to unset a cname before reseting it.
router/hipache/router.go: removed unused helper method
router/hipache/router_test.go: fixing redis connection related stuff
app/app_test.go: provisioning app before unsetting its cname