Add scenario test for image service

As the key component of OpenStack, it would be great if Glance can be covered by scenario test in Tempest. From the image service perspective, below test cases should be included.

     * create a new image without data
     * list images to see if the new image is there or not
     * upload image data against the new image
     * create an instance and make sure it works
     * create another tenant
     * share the image with the 2nd tenant
     * create an instance in the new tenant based on the image and make sure
       it works
     * unshare the image
     * make sure it's invisible from the new tenant
     * make the image public
     * ensure the public image can be seen and used from the 2nd tenant