Add nginx.gracefulrestart

This calls `nginx.gracefulstop` (which blocks until Nginx has finished
in-flight requests) and then calls `nginx.start` before moving onto the next
host.

I need this to rollout some Nginx config changes that aren't picked up by a
graceful reload (HUP). It can also be used to pick up OpenSSL lib upgrades.
Merge pull request #52 from alphagov/nginx_gracefulrestart

Add nginx.gracefulrestart