provider/maas: implement zone distribution/placement support
provider/maas: add tests for AZs
Update dependencies.tsv
Fix common AZ support code

We should not bail out of there are no instances,
we should just handle ErrNoInstances gracefully.

Also, fix a recently merged test.
provider/common: fix more common code
Merge pull request #847 from axw/lp1360605-maas-zones

provider/maas: implement zone distribution/placement support

This commit introduces support for "zone=" placement,
and automatic distribution across physical zones in
MAAS. We treat MAAS' physical zones like availability
zones in OpenStack and EC2.

Fixes https://bugs.launchpad.net/juju-core/+bug/1360605