This is analagous of the bwc-zip for elasticsearch. The one caveat is
due to the structure of how ES+xpack must be checked out, we end up with
a third clone of elasticsearch (the second being in :distribution:bwc-zip).
But the rolling upgrade integ test passes with this change.
relates elastic/x-pack-elasticsearch#870
Original commit: elastic/x-pack-elasticsearch@34bdce6e99
Ryan Ernst