Support macaroons for migration authentication

[mjs]

Up until now migrations have only supported password authentication to the target controller but with modern Juju most users will have a macaroon in their local account store and not a password. This PR threads support for macaroon authentication through the migration infrastructure so that users with macaroons can initiate migrations.

By reusing the user's macaroon directly, this implementation is a little naive. A upcoming change will have the client request a macaroon from the target controller, attenuate it and have that used for all future migration activities. Password authentication support for the migration infrastructure will be removed at that point - simplifying the code and improving security by not having long lived credentials being stored and passed around.

Drive-by: rename the GetMigrationStatus API on the MigrationMaster facade to MigrationStatus.

(Review request: http://reviews.vapour.ws/r/5498/)

[mjs]

$$merge$$

[jujubot]

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