use an icinga-specific default cgi.cfg file

Some configurations in there need to be adjusted for icinga.

We already have such a file so let's just point the sources to it.
icinga: remove inclusion of pnp4nagios.cfg file

we don't manage that config file in this place, and that actually
prevents the icinga daemon from starting.

if one wants to use pnp4nagios with icinga, then the nagios::pnp4nagios
class should be used in the same way that it would be with nagios.