Add support for configuration via env variables

All command line flags can be set via environment variables.
Given a cmd line flag like --etcd-endpoint, corresponding variable
becomes FLANNELD_ETCD_ENDPOINT. That is, FLANNELD_ prefix is appended
with upper case flag name that has dashes replaced with underscores.

Fixes #53
whatever happened to that new CLI framework that Shantanu was working on? I think that's the only place this would make sense - otherwise it's too little code to bother factoring out.