Add a new varnish_param type

It allows to modify varnish parameters where OSes put them by default:

* in /etc/default/varnish for Debian wheezy (as a list of parameters)
* in /etc/systemd/services/varnish.service for Debian jessie (as a list of
parameters)
* in /etc/sysconfig/varnish for RedHat 6 (as standalone variables)
* in /etc/varnish/varnish.params for RedHat 7 (as standalone variables)