fix cnames for etcd directories

if we put a hostname like 1.loadbalancer.skydns.local -> 127.0.0.1 in etcd, and
then put 1.backend.skydns.local -> loadbalancer.skydns.local, skydns wont
return any A records for an A query on backend.skydns.local. this commit makes
skydns handle cnames in etcd directories.