Sahara: minor changes for API tests

In file "test_cluster_templates.py" setUpClass method needs
to be wrapped in decorator "safe_setup" because resource
(node group template) is created in setUpClass method.

In addition small code refactoring in tests for data sources was made.

Partially implements: blueprint savanna-api-tests

Change-Id: If783df527fc23ea197c5344a67e1aa3e82774b2c
Merge "Sahara: minor changes for API tests"