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
Merge pull request #57 from eyakubovich/master

Add support for configuration via env variables