Use haproxy httpchk to detect a radosgw crash

TCP check by itself is only able to confirm that the Apache process is
still there and listening on port 6780, an HTTP GET request will get a
response code 500 when there is no radosgw process behind Apache, and
haproxy will mark such host as DOWN.

Change-Id: Iee8e02427d58befa51596867a5c4c623ee0ddbe8
Closes-Bug: #1385002
(cherry picked from commit 151ca375000faad75c92a709dee969f0caaeb18b)
Merge "Use haproxy httpchk to detect a radosgw crash" into stable/5.1