As fields with underscores will be disallowed in master, and we have to
prepare the upgrade, this commit renames the _status field to status.
When the 5.x upgrade logic is in place in the 5.x we can remove all the
old style _status handling from the master branch.
Note: All the BWC compatibility tests, that load 5.x indices are now
faking a finished upgrade by adding the `status` field to the mapping
of the watches index.
Original commit: elastic/x-pack-elasticsearch@9d5cc9aaec
lcawley
Alexander Reelsen