support consul encryption

[thraxil]

docker-consul currently can't be configured to join a cluster that uses gossip encryption: https://www.consul.io/docs/agent/encryption.html

[progrium]

Have you tried with the latest since moving to gliderlabs?

[ianmcorvidae]

It is possible to use gossip encryption with this image. Any files mounted into the /config directory will be picked up, so host-mounting a JSON file with the following structure will enable gossip encryption:

{"encrypt": "<token as generated by consul keygen>"}