Adding ETC binaries was a stop gap measure until packages were
available. With the impending release of bionic that'll be soon :)
Also using the mirror on tarballs.o.o was unreliable so these binaries
are cached in the nodepool images as described in. Having this
setting in d-g makes more work than is needed in the near term.
Removing it does introduce a new failure mode if an etcd version is
used in devstack but not yet cacched in the image.