Allow specifying the server_address option

I've added some simple logic to allow `server_address` to be specified in `site.pp`. This option allows the user to tell the NRPE service which interface to listen on (0.0.0.0 can be used to to listen on all of them, defaults to 127.0.0.1).

The "fixed it" commit (crap comment I know) fixes a minor syntax error where a "]" was missing  in `init.pp`.