allow more settings via environment variables

[costela]

Currently, setting some settings when using systemd (e.g. --flannel-iface) require editing the service file to add flags, which is slightly more cumbersome and less easy to automate than simply adding values to an envvar file.
It would be nice to be able to provide more settings via environment variables.

If there are no objections to this (or assuming I didn't miss some obvious reason why this isn't a good idea), I'd try my hand at a small PR.

[brandond]

Closing due to age. I believe all the args should have variables. If not, they can certainly be set using config file support.