Table hosts/service: Expose check_source

To be able to determine the source of the check, the check_source parameter is
needed to be available to the frontend. That contains either which worker
runned the check, or which merlin node.

Signed-off-by: Max Sikström <msikstrom@op5.com>
Filter out custom_variables that shouldn't be exporeted using prefix

To be able to use custom variables to store certain configuration parameters to
services and host checks, such as service account information, those variables
needs not to be exported to any GUI, but just be kept inside of nagios.

That means, those variables needs to be masked in the inteface between nagios
and the GUI, which for most cases is livestatus.

This patch adds the configuration line hidden_custom_var_prefix=PREFIX to the
broker_module line in nagios.cfg, which makes all custom variables with that
particular prefix hidden in the livestatus interface.

Should resolve a big part of #7894

Signed-off-by: Max Sikström <msikstrom@op5.com>
spec: Put op5's reserved hidden_custom_var_prefix in place on upgrades

I'll need to test this further on monday.

Signed-off-by: Robin Sonefors <robin.sonefors@op5.com>