Meter status made available to the uniter. The meter status is set on state and exposed to the charm
hooks in the form of environment variables. In addition, a new hook, meter-status-changed is executed
every time the meter status for that unit changes.
Merge pull request #817 from tasdomas/meter_status

Meter status

Meter status information available as hook context environment variables.

Meter status information will be set per-unit, once the metrics pipeline is in place.