Associate Harbormaster build target with leases

Summary: Ref T1049. This ensures the Harbormaster build target is associated with leases, so in the future we can query things and find out whether builds are still running with associated leases.

Test Plan: Leased a host, checked the DB and saw the field populated.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: joshuaspence, Korvin, epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10870