This is a specialized preloader for classes with polymorphic
relationships that allows for targeted preloading for those specific
class types on said polymorphic relationships.  This allows for taking
advantage of those associations by allowing scopes to be applied to the
specific relationships and subsequent calls to the relationship can have
those query definitions applied.

This makes a new rspec shared_context that can be used to replicate the
tree building functionality in the specs that test the method
`MiqPreloader.polymorphic_preload_for_child_classes`.  The same data can
then be used in higher level abstraction tests to confirm the
functionality works as expected there as well.

Makes use of MiqPreloader.polymorphic_preload_for_child_classes in
RelationshipMixin.fulltree_arranged to allow the caller to preload
specific resource relationships and sub relationships.

Implements the MiqPreloader.polymorphic_preload_for_child_classes
improvements into the MiqRequestWorkflow#ems_metadata_tree method.  This
significantly cuts down the number of N+1's when accessing the
v_total_vms method from Hosts via EmsClusters.

Also includes the `:v_total_vms` select to the individual hosts in the
ems_metadata_tree incase that method is accessed from those records as
well.