UUID generation change

* Moving PD and pushApp/variantID (UUID) generation into the model/entity classes;
* getting rid of hibernate annotations
* cleaning up code and tests
* added new test to test PK/ID generation

Was done for:
* [AGPUSH-382](https://issues.jboss.org/browse/AGPUSH-382)
* [AGPUSH-440](https://issues.jboss.org/browse/AGPUSH-440)