Add a type and provider to manage apt keys.

Add a type and provider to manage apt keys.

This commits introduces:
 * The apt_key type;
 * The apt_key provider;
 * Unit tests for the type;
 * Beaker/acceptance tests for the type/provider.

The idea behind apt_key is that apt::key will simply become a wrapper
that uses apt_key. Being a native type/provider apt_key is a lot less
error prone than the current exec behaviour of apt::key and adds a few
nice bonuses like inventory capabilities for mcollective users.