implementing backend selection

[PeterAlfonsLoch]

Pull Request (PR) description

Implementation of -> #31

"The current code for backend selection is broken with templates for vcl version 4.
It seems this fork has been created from an early state of maxchk's repository before all of this took place: https://github.com/maxchk/puppet-varnish/commits/develop/templates/includes/backendselection4.vcl.erb"

This Pull Request (PR) fixes the following issues

Jul 06 15:24:08 lb-ng varnishd[215185]: Message from VCC-compiler:
Jul 06 15:24:08 lb-ng varnishd[215185]: Expression has type directors.round_robin, expected BACKEND
Jul 06 15:24:08 lb-ng varnishd[215185]: ('/etc/varnish/includes/backendselection.vcl' Line 3 Pos 26) -- (Pos 28)
Jul 06 15:24:08 lb-ng varnishd[215185]:   set req.backend_hint = director_name;
Jul 06 15:24:08 lb-ng varnishd[215185]: -------------------------###-
Jul 06 15:24:08 lb-ng varnishd[215185]: Running VCC-compiler failed, exited with 2