Move build variables into HarbormasterBuildableInterface

Summary: Ref T1049. This moves the declaration of build variables onto HarbormasterBuildableInterface, allowing new classes implementing HarbormasterBuildableInterface to declare their own variables.

Test Plan: Implemented it on another class, saw the build variables appear.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D9618