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This commit adds support for CoreDNS through its etcd middleware.
Because the middleware is backward compatible with SkyDNS this
commit adds support for SkyDNS as well. In fact, new provider
is available under two names in CLI (coredns and skydns).

All interactions with middleware happen through etcd cluster,
whose location (URIs) is specified via --etcd CLI parameter
by default http://localhost:2379).

The provider translates CoreDNS/DkyDNS SRV records to
A/CNAME + optional TXT endpoints, when reading from etcd and
combines A/CNAME with TXT endpoints back into single SRV record
when writing it back.

Also adds github.com/coreos/etcd package to glide.yaml and vendor folder
because it is used by the provider