Allow Harbormaster to run commands on Drydock working copies

Summary: Ref T9252. This mostly cleans up future and log handling, and edges us closer to being able to do useful work with Harbormaster / Drydock.

Test Plan:

• Added a "Run ls -alh" step to my trivial build plan.
• Ran it a bunch of times.
• Worked great.
• Also did an HTTP plan.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14161