This adds --tag to the v2 version of `image set`. This
is another step to compatability between the osc image api.
Added merge of tags into existing tags and handling duplicates,
and tests for same.
Co-Authored-By: Steve Martinelli <stevemar@ca.ibm.com>
Change-Id: Ie800fcbf8bbc0978c54ace3278750a18023e8ce4