Add ssl/tls support in discovery backends

[david-guenault]

Since all the docker tools support SSL/TLS (docker, swarm, compose ...) should it be possible in the future to add the same for discovery backends. For exemple etcd and consul does support ssl/tls but it is not possible to enable it in swarm discovery backends.

As a reference you can see #360

[vieux]

I agree

[david-guenault]

@vieux, thinking about that i do not see how it should be implemented. The only parameter that it is curently possible is the discovery uri in the form [backend]://ip:port, ip:port,.../path. How should it be implemented ? With environment variables ?

[vieux]

@david-guenault what about etcds:// and consuls:// ? and we use the same tls certs we already have

[thaJeztah]

or something like etcd+https://, consul+https://?

[bacongobbler]

I'm 👍 for etcds://. I could see etcd+https:// being interpreted by users as being two separate connections (etcd and https) rather than a single connection through a tls-secured etcd backend. https:// also sets a precedent here as being "HTTP over TLS", so it makes sense in my mind that etcds:// implies "etcd over TLS".

Plus, it's less typing :)

[david-guenault]

sorry i've been out for a while. Well etcds seems fine for me.

[abronan]

Fixed by #1254. Swarm now supports TLS for consul and etcd through libkv and the --discovery-opt flag. Closing this one.