This adds an improved apt configuration format that is fully backwards
compatible with previous behavior. This is mostly copied from curtin's
* clean up and centralizes many of the top level 'apt_*' values that
previously existed into a single top level 'apt'key.
* support a 'source' in apt/sources/entry that has only a key
* documents new features and adds tests.
See the added doc/examples/cloud-config-apt.txt for more information.