Sahara: preparations for new tests

* Method "create_node_group_template" was slightly changed;
* New methods create/get/list/delete for cluster templates to API client were added;
* Method "create_cluster_template" was added;
* tearDownClass method was expanded;
* Some code formatting was done.

Partially implements: blueprint savanna-api-tests

Change-Id: Icbbe2295803ffba8d13b1af2c7fbae2c7e5cb4d4
Merge "Sahara: preparations for new tests"