Introduce apt-mirror config

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/