Adds tests similar to `:qa:full-cluster-restart` for x-pack. You
run them with `gradle :x-pack:qa:full-cluster-restart:check`.
The actual tests are as basic as it gets: create a doc and load it,
shut down, upgrade to master, startup, and load it. Create a user
and load it, shut down, upgrade to master, startup, and load it.
Relates to elastic/x-pack-elasticsearch#1629
Original commit: elastic/x-pack-elasticsearch@8994bec8e7
d526461bd2