Move integration tests under mistralclient/tests folder

- Collect all tests in one folder 'mistralclient/tests'.
- Create new folder 'functional' in the mistralclient/tests for integration functional tests.
- Restructure tests, create folder 'v1' for current tests (version 1).
- Create new folder 'unit' in the mistralclient/tests and put all unit tests here.
- Correct imports in the file with unit tests.

Change-Id: If92c11640d7cb61ee8ffcdc7ba99d7ab1061afa7