Support runtime extension of the MK image

This adds support for exposing an extension zip file to the MK image.
This will be downloaded, if present, and unpacked to allow runtime extension
of the MK without having to rebuild the ISO image.

This only supports a single zip file, putting responsibility on the user to
manage updates in a safe, effective fashion.

https://tickets.puppetlabs.com/browse/RAZOR-42
Signed-off-by: Daniel Pittman <daniel@rimspace.net>