Added support to manage bans and params

* Added support for ban and ban.list
* Added support for param and param.show
* Added support to obtain complete version of varnishd (incl commit
  hash)
* Simplified purge function to use recommended Varnish 4.x syntax that
  actually works in Varnish 3.x
* Dropped support for Varnish 2.x in purge (actually it's pretty old and
  unrecommeded)
Fixed some little things to satisfy pylint
Fixed some more little things to satisfy pylint
Merge pull request #14219 from moreda/topic/improve-varnish-module

Added support to manage bans and params