Each container on the Docker servers, when associated to an HTTP
host in the configuration, should serve a /502.html from rDII.
This change provides a class to read the rOPS configuration,
extract the domains names and check https://<host>/502.html.
When the page exists, it should return a 200 (502 is sent by the
web server on error, direct request of this page is a regular 200.)
Ref T114.