Use `mask` option for defining subnet on network configuration. Allow
options to be passed through using hash scoping and docker_network and
docker_connect prefixes. Enable public networks. Allow configuration
to define pre-existing networks by name.
This commit updates the behavior of how the docker provider creates new
docker networks. It looks at each existing network to see if the
requested subnet has already been configured in the docker engine. If
so, Vagrant will use that network rather than creating a new one. This
includes networks not created by Vagrant. Vagrant will not clean up
these networks if created outside of Vagrant.
Chris Roberts
Brian Cain