Add 'event-sample-overflow' option to charm config parameters

[James Hilling]

On many occasions, we have to set the following within the LXD containers for rabbitmq-server:

$ sudo rabbitmqctl set_policy -p openstack $POLICY_NAME '$QUEUE_REGEX' '{"max-length": $INTEGER_VALUE }' --apply-to queues --priority 1

We do this in order to limit the amount of messages that can exists within a specific queue.

It would be useful if there was a way to set this from the charm parameters.

Example:

sudo rabbitmqctl set_policy -p openstack event-sample-overflow '^event.sample$' '{"max-length": 99 }' --apply-to queues --priority 1