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.
Merge pull request #81 from mal/COOK-2280

COOK-2280: Allow custom timing of reload after nginx_site