provider/maas: stop retrying AZs when one succeeds

Derp. Forgot to break out of loop on success.

Fixes https://bugs.launchpad.net/juju-core/+bug/1392411
Merge pull request #1138 from axw/lp1392411-maas-azretry-break-1.21

provider/maas: stop retrying AZs when one succeeds

Derp. Forgot to break out of loop on success.

(backport)

Fixes https://bugs.launchpad.net/juju-core/+bug/1392411