Refactor Images API v2.0 client

Refactors 'store_image_file' method signator:
  - added content type as a new parameter in order to test for
    upload of image file with a different (from the default 'application/octet-stream')
    content type.

Change-Id: Id7cfc6cc73e5f7cb25e579996903efce35d0c67a
Merge "Refactor Images API v2.0 client"