COOK-2280: Allow custom timing of reload after nginx_site

Allows the reload notification to be triggered immediately in situations where that's useful. For instance where nginx is installed near the start of a chef run, and the default site needs to be disabled quickly or other sites need to be made active ASAP.

Retains identical default behavior.

Fixes: [COOK-2280](http://tickets.opscode.com/browse/COOK-2280)