Introducing AutoCompleteCascadeSelectField to ServiceCatalogMixin.

It is now possible to make 'cascading selects', i.e. narrowing the results of one autocomplete field according to the value selected in the other (parent/child relationship).

This concept is introduced here with `ServiceCatalogMixin`, i.e. the possible completions for device environment field are presented according to the value selected in the service catalog field.