unattended-upgrades template enhancement

I have collapsed the three `unattended-upgrades.*.erb` templates into one, and moved the distribution-specific logic into the `apt::unattended-upgrades::automatic` and `apt::params` classes.  This enables a user to pass an "extrarepos" parameter to `apt::unattended-upgrades::automatic`, which will allow unattended upgrades from additional origins.

If you're still not using parameterized classes, you will probably need to cherry-pick some of these commits.