/etc/rabbitmq not readable by rabbit

[Hugh Saunders]

Rabbit failed to start during an upgrade from Juno to Kilo.

The error in the startup log was:

---------------------------------------------
BOOT FAILED
===========

Error description:
   {error,
       {cannot_read_enabled_plugins_file,"/etc/rabbitmq/enabled_plugins",
           eacces}}
---------------------------------------------

Ther perms on /etc/rabbitmq are root:root 0750, which doesn't allow the rabbitmq user to read.

The perms are set in pre_install to 0755: https://github.com/openstack/openstack-ansible/blob/master/playbooks/roles/rabbitmq_server/tasks/rabbitmq_pre_install.yml#L54

The perms are overwritten by the memcached module to 0750 here: https://github.com/openstack/openstack-ansible/blob/master/playbooks/roles/rabbitmq_server/tasks/rabbitmq_ssl_key_distribute.yml#L26-L27

I think the solution is to remove the dir_mode from "Distribute self signed ssl" then the memcached module will use its default of 0755.

This is related to bug #1506992