Add publisher infrastructure

Adds plumbing for "publishers."  Publishers are strategy classes
responsible for preparing images for distribution.

An example publisher should be a BitTorrent publisher that took the
image binary, generated a torrent file from it, and started an initial
seeder process.

The default publisher is nop.Publisher, which takes no action.

Change-Id: I8fd952270aaf4fc6962dafc1f287aca0cd0c9828