Add LXC build step

Summary:
This adds a new abstraction using the changes-lxc-wrapper helper CLI
(changes-lxc) to be entirely powered based on commands via the master,
rather than proxying through changes-lxc-wrapper.

Reviewers: vishal

Reviewed By: vishal

Subscribers: rajat

Differential Revision: https://tails.corp.dropbox.com/D65980