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.
Reviewed By: btrahan
Maniphest Tasks: T2015
Differential Revision: https://secure.phabricator.com/D3999