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
Yeah, we go back and forth on this - previously we've used libraries for these things but over the last months have trended towards removing them out and just using simpler versions of our own.