Fix rabbitmq perms

If a bootstrap fails at some stage and leaves a rabbitmq_data
container on a host, it will not bootstrap again and rabbitmq
fails to start due to root permissions on /var/lib/rabbitmq.

This patch improves the check for a valid, existing
rabbitmq_data

Closes-Bug: #1490602
Change-Id: I6a00aaef38b4ab6a60dcfb5ed2f808513b9f8c84

ansible/roles/rabbitmq/tasks/bootstrap.yml

 ---
 # TODO(SamYaple): Improve check with a custom docker module to check if alive
 - name: Checking if a rabbitmq_data exists
-  command: docker inspect rabbitmq_data
+  command: docker exec rabbitmq_data stat /var/lib/rabbitmq/.erlang.cookie
   register: exists
   changed_when: False
   failed_when: False