Add etcd3 support for CoreDNS provider

[bryanalves]

The current CoreDNS provider writes entries to etcd using the v2 api. CoreDNS starting in version 1.2 started using the v3 api:

https://github.com/coredns/coredns/tree/master/plugin/etcd#migration-to-etcdv3-api

There's a couple of options here:

1. Add a config parameter to enable etcd2 or etcd3 mode
2. Add an environment parameter (similar to ETCD_URLS) to do the same
3. Create a new provider altogether (if the API is sufficiently different, which it might be)

[fturib]

does it match this already opened PR ? #686

[bryanalves]

Yep, that seems to be the case. I totally missed that when searching somehow!