auto-negative tests

[maxlinc]

Tempest (the OpenStack integration test suite) has the ability to take a schema and automatically generate a negative test scenario - by altering a field to be invalid and making sure an error happens.

It'd be nice to at least use this approach in Pacto's own unit or integration testing, if not as an actual feature.

See:
http://docs.openstack.org/developer/tempest/HACKING.html#negative-tests
https://github.com/openstack/tempest/blob/master/tempest/common/generator/negative_generator.py
https://github.com/openstack/tempest/blob/master/tempest/api/compute/images/test_images_negative.py