R1:cce5ebebe98b

Improve Drydock's ability to allocate leases correctly

Summary:
Right now, Drydock gives out multiple leases to the same working copy and gives out leases to working copies with repository "P" in them when the user requested some other repository.

Add two callbacks:

- `canAllocateLease()` - allows a blueprint to reject a lease on a resource because of a fundamental incompatibility, like "it's a working copy with Phabricator in it, but the lease wants a working copy with Javelin in it".