Fix the rabbitmq-limits validations

The validation fails because the rabbitmqctl command needs to be run as
root. This adds "become: true" to the task to fix it.

Change-Id: I565119bcfd27460cda1992d8922eba500a5db157
Closes-Bug: #1637260

validations/rabbitmq-limits.yaml

@@ -10,6 +10,7 @@
     min_fd_limit: 16384
   tasks:
   - name: Get file_descriptors total_limit
+    become: true
     register: actual_fd_limit
     command: "rabbitmqctl eval 'proplists:get_value(max_fds, erlang:system_info(check_io)).'"
     changed_when: false