Sahara: minor changes for API tests

* Code refactoring of "test_node_group_template_create" test
  (file test_node_group_templates.py) was done.
* Unnecessary blank lines were removed.

Partially implements: blueprint savanna-api-tests

Change-Id: Id21edafcc4d1d0345b72404a55bbf9d69618cd95
Merge "Sahara: minor changes for API tests"