Small refactoring for Sahara platform test

* Method "_test_image" was renamed into "check_image"
  because we check image but not test

* Steps to list node group templates and cluster templates were removed
  because sanity tests have these checks

* Some messages were corrected

* Sahara platform test was renamed

Change-Id: I5ca9c25ed2e04eca09c8632a6e2757706990b2f9