Hard restart Varnish on config changes

Because reload doesn't clear out old backends. We have to use 'stop;start'
because the init script doesn't propagate the correct exit code from
'restart', even though it calls the same thing. The default of `logoutput =>
on_failure` is fine.
Merge pull request #44 from alphagov/hard_restart_varnish

Hard restart Varnish on config changes