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.
This commit changes the way ports are aggregated in the Docker provider.
Previously ports were aggregated by their "number", but that is not a
truly unique representation. Instead, the protocol is now taken into
account when generating the port map.
Fixes GH-5527
