properly flatten the bits backend list

puppet is expecting a list and gets an hash:

err: Could not retrieve catalog from remote server: Error 400 on SERVER:
comparison of String with Hash failed at
/etc/puppet/modules/varnish/manifests/instance.pp:37 on node cp1056.eqiad.wmnet

RT #6794

Change-Id: Ib6cb020bed50b3209be8a9001396bfa5ecd5ef5e