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/517545
Committed: https://git.openstack.org/cgit/openstack/charm-swift-storage/commit/?id=0876b4fb73a9b0a06f514416af7f4fd70519b141
Submitter: Zuul
Branch: master

commit 0876b4fb73a9b0a06f514416af7f4fd70519b141
Author: James Hebden <email address hidden>
Date: Fri Nov 3 16:35:11 2017 +1100

    Add statsd metrics support to the swift-storage charm

    By default, statsd metrics can be sent by the swift account,
    container and object storage services 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: #1729770

    Change-Id: If1bf3ced8a9ed07af81f352eb0263659d147e3aa