Add a --target flag to arc unit for uploading results
Summary: Ref T5821. Basic idea here is that Harbormaster can run arc unit --everything --target ... to get a build target updated.
Test Plan: Ran arc unit --target ..., saw web UI update with test results.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T5821
Differential Revision: https://secure.phabricator.com/D14190