heat's tempest tests need to be able to create resources for themselves

currently Heat's tests depend on some pre-created resources - networks, flavors and images.

While requiring a special image is fine, both the network+subnet+router and flavors can be created
by tempest tests themselves during a test run, and clean them up in the end.

This will allow decoupling of heat tempest plugin from many configuration options it does have now,
including explicit user credentials to use (users can be created on the fly as most other tempest tests do).