Sahara: preparations for data source tests

* create/list/get/delete methods to API client for data sources were added;
* Method "create_data_source" was added;
* tearDownClass method was expanded;

Partially implements: blueprint savanna-api-tests

Change-Id: I60d7a5553b642a2a7a03d4c6cf2befdfe2172341
Merge "Sahara: preparations for data source tests"