Enable client V2 to update/delete tags for a given image.

Added the CLI option image-tag-update to associate a tag to an image via API V2.
Added the CLI option image-tag-delete to delete a tag associated with an image via API V2.

Related to bp glance-client-v2

Change-Id: I76060e1982223770a6c2c0bd9376d568af0df456