Add availability zones to instance data.

* Add the instance availability zone name to state.instanceData.
* Add an upgrade step for 1.22 that sets the AZ on state.instanceData for any instance missing it (or to "" where the provider doesn't support zones).
* Add the availability zone to a couple key structs (MachineTemplate and StartInstanceResult).
* Set the availability zone in StartInstanceResult for providers that support zones.
* Add an availability zone param to a few method signatures and API param structs (e.g. SetInstanceInfo, SetProvisioned).

(Review request: http://reviews.vapour.ws/r/557/)