Allow Drydock to allocate (very basic) working copy resources

Summary: This is missing a lot of features, but technically allows working copy allocation.

Test Plan: Ran drydock lease --type working-copy --attributes repositoryID=12, got a working copy of Phabricator allocated on disk.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D3999