Use of single interface for creating image

Within scenario manager, we had two methods for creating image
those are as follows:
1. _image_create() - private interface which creates image
2. glance_image_create() - public interface which uses the above
   method eventually to create image

Hence, let's have single method for single work. Thus, we need to
replace '_image_create()' with 'image_create()'.

Implements: blueprint tempest-scenario-manager-stable
Signed-off by: Soniya Vyas<svyas@redhat.com>
Change-Id: I33e6abd416bbe5964a279f7969615ffd2974b4dd
3 files changed