Consistent naming scheme; rabbitmq_host -> rabbitmq_hostname

environment.sh uses hostname for everything, and both environment and
credentials provide a default of 'rabbitmq', so this should be one less
variable to care about.

installer/roles/local_docker/templates/credentials.py.j2

@@ -13,7 +13,7 @@ DATABASES = {
 BROKER_URL = 'amqp://{}:{}@{}:{}/{}'.format(
     "{{ rabbitmq_user }}",
     "{{ rabbitmq_password }}",
-    "{{ rabbitmq_host | default('rabbitmq')}}",
+    "{{ rabbitmq_hostname | default('rabbitmq')}}",
     "{{ rabbitmq_port }}",
     "{{ rabbitmq_default_vhost }}")