Sahara: add API tests for data sources

* create/list/get/delete API tests for data sources in Swift were added
* create/list/get/delete API tests for data sources in local HDFS
  were added
* create/list/get/delete API tests for data sources in external HDFS
  were added

Partially implements: blueprint savanna-api-tests

Change-Id: I78fe5c53d3af84c04402966891c096c8cbd1253a
Merge "Sahara: add API tests for data sources"