restructure varnish::instance's "directors"

XXX WIP XXX

Switches each directors data item from [backends] to a hash, one
element of which is 'backends' => [backends].

Moves per-instance "director_type" inside of each director's hash
as key 'type'.

Adds per-director hash key 'dynamic' for etcd stuff.

Change-Id: Ib2c7d82ac8339203daf3189473024a860ececd2a