Add a service restart parameter.

This PR creates a parameter for a service restart command.

One use case is using `named-checkconf -z /etc/named.conf && service named restart` as the restart command to prevent the module from restarting BIND if there are syntax errors in `named.conf` or any of the zone files `named.conf` loads.

The defaults are set to `service ${servicename} reload`, which is what was hard-coded into `manifests/service.pp` before.