Refactor secret functional tests using models and behaviors

Updated the functional tests to use models and behaviors for secrets.
* Adding simple HATEOS-compatible rest client
* Fixing model de/serialization
* Modifying all tests to conform to the new client
* Adding option to load tempest config from local etc
* Incorporating review feedback

Change-Id: I497b4f7bf10a9f47ca399b569d964762505466c9
Merge "Refactor secret functional tests using models and behaviors"