Support for local external port configuration

Add's ensure_local_external_port which sets up an external gateway
tap the same way we do for the management network.

New config options
- external_subnet_id
- external_prefix (default 172.16.77.0/24)
- plug_external_port (default False)
Add new config options to rug.ini
code cleanups
more format fixes
flake8 fixes
Merge pull request #186 from jordant/master

Support for local external port configuration