Set Berksfile to use ruby solver

This provides much better debugging output.

Depends-On: https://review.opendev.org/702772
Change-Id: I1a738e6f951b12b54bbc2eaf61d5b01ab70a1203

Berksfile

@@ -1,5 +1,7 @@
 source 'https://supermarket.chef.io'
 
+solver :ruby, :required
+
 %w(common).each do |cookbook|
   if Dir.exist?("../cookbook-openstack-#{cookbook}")
     cookbook "openstack-#{cookbook}", path: "../cookbook-openstack-#{cookbook}"