maas: fix availability zone selection
maas: add tests for availability zone selection
provider/maas: minor cleanup of availability zone loop
provider/maas: update conditional continue
provider/maas: explict nil since err might be set
provider/maas: fixing test and continue conditional
Merge pull request #1157 from wwitzel3/ww3-lp-1392390

provider/maas: fixes lp:1392390

Fix for loop used for maas availability zone selection.
Add tests for maas availability zone selection.

  * Bug: https://bugs.launchpad.net/juju-core/+bug/1392390