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