Added arg and config option to manually set gw_mode.
When gateway_mode is auto, a temporary config will be added, and after a
ten sec delay, naxalnet will check if systemd-networkd shows a
'routable' connection. If the connection is routable, gw_mode will be
set to server; or client otherwise.
We now use NetworkD() to configure systemd-networkd.
Path.glob doesn't return things in alphabetical order, which is needed
to configure networkd without any error. Need a fix for that.
Too much configs and args make debugging harder. The class networkd will
be used to add config files to the networkd runtime dir after formatting
the file with str.format().
