Add the API entry point for entity zones.
Add a test for InstanceId.
Add a test for Zone in the API server.
Add a test for Zone in the API client.
Prep unit-get for more settings.
Add "zone" to unit-get.
Add the Zone method to Context, using the API end-point.
Zone -> AvailabilityZone.
In-line an error test.
Use errors.
Use a fixed-literal syntax.
Be more rigorous about checking the zones we get back.
Add a note about not validating the returned zone names.
Add notes and doc comments for the code added to the dummy provider.
zone -> availability-zone.
Use an empty string to indicate that no zone name was found.
Use errors.Trace.
Drop the new command doc.
Fix tests.
Factor out code for finding availability zones.