Bug#1039475: icingaweb2 backend is not configured correctly

[Guido Berhoerster <guido@berhoerster.name>]

The documentation no longer mentions "config_backend" in the global
configuration (see
https://icinga.com/docs/icinga-web/2.11/doc/03-Configuration/#global-configuration)
and the changelog mentions that "user preferences" are now stored in the
DB (see
https://icinga.com/docs/icinga-web/latest/doc/80-Upgrading/#upgrading-to-icinga-web-211x)
though from the tracking bug it is not immediately clear what goes into
the ini files and what is stored in the DB (see
https://github.com/Icinga/icingaweb2/issues/3770).

Debugging
Icinga\Module\Monitoring\Backend\MonitoringBackend:loadConfig() confirms
that the backends ($backends) configuration is actually empty, i.e. that
no backends are defined in the configuration (see
https://github.com/Icinga/icingaweb2/blob/v2.11.4/modules/monitoring/library/Monitoring/Backend/MonitoringBackend.php#L160
for context).

-- 
Guido Berhoerster