state.py is responsible for collecting and storing the network
configuration. It will also be responsible for detecting differences
between the system's and netplan configuration in the future.

The main motivation for this new submodule is the centralization of
state management.
The code used by "netplan status" to read and aggregate the system's
network configuration and the code used by "netplan get" to read the
Netplan's configuration was moved to this module.

Make use of the new module to read the configuration used by "netplan
get" and "netplan status".

It enables us to retrieve the list of IPs, (V4 and V6) from a given
netdef via Python bindings.

The same iterator will consume the ip4_addresses, ip6_addresses and
address_options (where IPs with options are stored).

Each item returned by the _next() function is a NetplanAddressOptions*,
even for addresses without options. Each object is released when the
next one is requested.