Add config.yaml entires for statsd logging configuration

[James Hebden]

Swift is able to log metrics via statsd for diagnostic and monitoring purposes.
The log_statsd_host and log_statsd_port can not currently be set using this charm, which are the settings required to enable statsd logging.
Opening this bug to track this feature request, I have a review and patch on the way to add this.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/517543
Committed: https://git.openstack.org/cgit/openstack/charm-swift-proxy/commit/?id=a11ff79fc7900879fa077e92c567341305964716
Submitter: Zuul
Branch: master

commit a11ff79fc7900879fa077e92c567341305964716
Author: James Hebden <email address hidden>
Date: Fri Nov 3 14:58:24 2017 +1100

    Add statsd logging support to the swift-proxy charm

    By default, statsd metrics can be sent by swift-proxy
    for diagnostic and monitoring purposes, but are disabled
    by default. This change exposes charm config settings
    that allow it to be enabled by setting 'statsd_host'
    to a non-empty value. 'statsd_port' and 'statsd_sample_rate'
    are also supported for changing the destination port
    and rate at which metrics are collected.

    Closes-Bug: #1729771

    Change-Id: I2d5cca233d48022073b5975c06c3da2b1896d8d9