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
0876b4fb73