Create an aptly::mirror defined type

Per the class docs:

Create a mirror using `aptly mirror create`. It will not update, snapshot,
or publish the mirror for you, because it will take a long time and it
doesn't make sense to schedule these actions frequenly in Puppet.

It will also import the GPG key into the keyring that aptly defaults to.
Merge pull request #2 from alphagov/mirror_defined_type

Create an aptly::mirror defined type