* etcd service containers usually have a set of
environment parameters required to boot the container.
* The short-lived etcd bootstrap containers pass extra
ETCD_INITIAL_* environment variables, but still need to
pass the ones that the service containers use.
* This uses ansible's `combine` filter to cut down on the
duplication.
* This is intended to be just a straightforward refactor.
Change-Id: I04e95f92a8f365553afd618d58b99de595d48312
ae21f31785