_wait_for_completion defaults to false. If set to true then the API will
wait for all the tasks that it finds to stop running before returning. You
can use the timeout parameter to prevent it from waiting forever. If you
don't set a timeout parameter it'll default to 30 seconds.
Also adds a log message to rest tests if any tasks overrun the test. This
is just a log (instead of failing the test) because lots of tasks are run
by the cluster on its own and they shouldn't cause the test to fail. Things
like fetching disk usage from the other nodes, for example.
Switches the request to getter/setter style methods as we're going that
way in the Elasticsearch code base. Reindex is all getter/setter style.
Closes#16906
