Add ability to flush varnish. Flush varnish on deploy (inside our deploy...

This is to address https://github.com/geoadmin/mf-geoadmin3/issues/1920

Note that flushing varnish is done inside our deploy scripts. So this does not work if you deploy by hand.

See the README about the deployment.

(We can't put it in the hook as the deploy user can't access the varnish instances)