We introduce a new apt-mirror config attribute,

and propagate it through to environs/cloudinit.
Bootstrap machines and manually provisioned
machines are configured using the updated
cloudinit/sshinit, and other machines are updated
using the standard apt_mirror cloudinit config.

Fixes https://bugs.launchpad.net/juju-core/+bug/1364200

http://reviews.vapour.ws/r/157/