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