WebStyle: ping handler returning 200 status code

• When queried by a load-balancer (such as haproxy), this commits makes the /ping web handler to return HTTP status code 200, even if e.g. ACCESS_CONTROL_LEVEL_SITE is set to 1 (which implies returning HTTP error status code 503 for any other web handler so that crawlers don't register these requests). (PR #2700)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>