Expose more rabbitmq tunables via config

[Edward Hope-Morley]

There are a number of useful tunables that are not currently exposed by the
charm but that would be of benefit to those running production clusters on
machines with above-standard resource quotas for which the current defaults
do not necessarily apply.

I would like to propose that we expose the following via the charm config:

  * VM I/O Thread Pool (set via RABBITMQ_SERVER_ADDITIONAL_ERL_ARGS in [1])

    The default value has increased in newer versions but in Xenial
    (rabbitmq version v3.5.7) it is 30 which, according to the upstream
    documentation [0], is non-optimal since the recommendation is to use
    a value no lower than 96 when you have >= 8 cores (i.e. min 12 threads
    per core) - which is more than likely the case for most production
    environments.

  * Connection backlog (set in tcp_listen_options in [2])

    Again this value defaults to 128 which, for larger deployments with a
    large number of clients and particularly where network interrupts are
    extant and reconnects undesirable this number should be higher. Docs
    suggest going up to 4096.

[0] https://www.rabbitmq.com/networking.html
[1] /etc/rabbitmq/rabbitmq-env.conf
[2] /etc/rabbitmq/rabbitmq.config

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/469141

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/469141
Committed: https://git.openstack.org/cgit/openstack/charm-rabbitmq-server/commit/?id=96fa17720b7fb16eadd5531c88d20005a0425d31
Submitter: Jenkins
Branch: master

commit 96fa17720b7fb16eadd5531c88d20005a0425d31
Author: Edward Hope-Morley <email address hidden>
Date: Tue May 30 15:13:26 2017 +0100

    Expose performance tuning options via charm

    Adds connection-backlog and erl-vm-io-thread-multiplier config
    options which are required for tuning when deploying in
    environments with large numbers of clients and server hosts
    with high numbers of cores respectively. Also refactors
    rabbitmq-env.conf rendering code so that it is properly
    applied by the charm.

    Change-Id: I8596eb9a0419d4e64782bfaf9d8c0f67e5de96b1
    Closes-Bug: 1693561