Introducing Environ.ListNetworks()

Provides a way for juju to discover what networks
are available from the provider. Later, we'll use
that information to discover networks initially
(at bootstrap) and to update them when they change.

Implemented in dummy provider only and stubbed it
out in the others

In a follow-up a concept of "pending networks" will
be introduced in state to hold such networks until
the user decides to use them (i.e. by running
juju add-network --existing).