Update ETCD_DOWNLOAD_URL
Update ETCD_DOWNLOAD_URL

Looks like https://github.com/coreos/etcd is redirecting to
https://github.com/etcd-io/etcd, so let's use that official address as
the download URL.

Change-Id: I39355b4deb17ae11927a5339e73fb8ec3a274cf7

group-policy/devstack,openstack-dev/devstack,noironetworks/devstack,group-policy/devstack,nawawi/openstack,noironetworks/devstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack