Pass full Harbormaster URIs to Buildkite

Summary: See PHI611 for details.

Test Plan:
Ran a Buildkite build, saw Buildkite confirm receipt of these parameters in the HTTP response:

{F5562054}

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19419