Implement support for leasing from Drydock hosts in Harbormaster

Summary:
This adds LeaseHostBuildStepImplementation for getting leases on hosts in Drydock via Harbormaster. It stores the resulting lease in an artifact.

There is also a few bug fixes as well.

Test Plan: Created a build plan with a "Lease Host" build step. Ran the build plan and saw the build pass and the artifact in the database.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049, T4111

Differential Revision: https://secure.phabricator.com/D7706