add datacat_collector collects parameter

This allows collectors to declare interest in multiple sources of exported
fragment, for a many-to-many system eg

   datacat { "service::user ${username}":
      path => "/home/${username}/.service",
      collects => 'service::user',
      template => 'service/config.erb',
   }

Common user config can be exported as per all users, and also fragments for
"service::user ${username}" (with a higher priority)
Merge branch 'collects_parameter'
What we done did