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.

sorry there is no test. it wasn't immediately obvious if the right records are in place to just add a test to the array, or if i need to add new records. i can try to write a test before the pr is merged if you prefer.