Allow Directory tree service to use other schemas than vocabulary and xvocabulary

Make the Directory tree service check for the presence of the 'parent' field instead of checking 'xvocabulary' schema.
This will allow to use other schemas than vocabulary and xvocabulary, the schema must have a 'parent' field when using it for a hierarchical vocabulary.
We assume that the schema has id, label, order and obsolete fields.

The use case covered by this is to be able to use schemas that "extend" 'vocabulary' or 'xvocabulary', so with the same set of fields, with one or more additional fields for specific stuff.

Needed by multi tenancy where we have custom schemas with one more field, tenantId.

For more configurability on the Directory tree service, see NXP-7174.