Sahara: preparations for job tests

* create/list/get/delete methods to Sahara API client for jobs
  were added
* Method "create_job" was added
* tearDownClass method was expanded

Partially implements: blueprint savanna-api-tests

Change-Id: I8d52a7505aeb25b7de3f6c8479daa056b8c6eae7
Merge "Sahara: preparations for job tests"