migrations (gce): Extract zone name from URL for SetMetadata

[babbageclunk]

Description of change

Fix the code that updates the controller tag of machines in the model -
otherwise migrations would fail on GCE. The GCE API requires the
unqualified zone name, so "us-central1-a" rather than:
https://www.googleapis.com/compute/v1/projects/gothic-list-89514/zones/us-central1-a
(Which is what the API gives back to us.)

QA steps

• Bootstrap 2 GCE controllers, A and B.
• Add a model to A and deploy an application to it.
• Migrate model A:m to B.
• The migration to B should finish successfully and the model should be fully removed from A.
• Destroying controller A should leave the machines in B:m running.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1667172

[babbageclunk]

Cheers!

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju