Adds api test to test_images

This adds a testcase to test api's
get image by id
get image file
also adds necessary client method for image client.

There was a confussion in the V2 imageclient function
get_image_metadata which has been renamed to get_image
to unify naming convention with V1. So there was change
made in test_images_tags.

Change-Id: Ie96135973b5796652e28aed2e3a09c17cfd26229
Merge "Adds api test to test_images"