OPENDJ-3246 Support polymorphism in CREST API and OpenAPI descriptors

Addressing review comments from previous pull requests.

Resources.java:
In buildJsonSchema(), add the "discriminator" property to the JSON schema
 and if a superType exists, then return a JSON schema using the "allOf" property.
In definitions(), output all the resources from the type hierarchy: all superTypes and subTypes
Replaced all uses of "allProperties" field by "declaredProperties" field when dealing with API descriptor.

ResourceTypePropetyMapper.java:
In isRequired(), now return true because the discriminator is a required property in JSON schemas.

opendj-bom/pom.xml:
Need to tmporarily depend on SNAPSHOTS for forgerock-commons
 until all work on forgerock-commons is finished and a new release exists.