Add command and test expansion APIs

Summary:
This adds the "expand jobstep" endpoint (/jobsteps/:jobstep_id/expand/) which is responsible for taking a payload similar to the previous Jenkins artifacts and expanding them into child jobsteps.

Currently it only supports the DefaultBuildStep implementation (which just changes the state to allocated).

Fixes GH-21

Reviewers: vishal

Reviewed By: vishal

Subscribers: rajat

Differential Revision: https://tails.corp.dropbox.com/D61253