Fix glance V2 incorrectly implements JSON Patch'add'

According to the RFC for JSON Patch
http://tools.ietf.org/html/rfc6902#section-4.1
the 'add' operation, in the event that the property being
added already exists, should replace the existing value. For backward
compatibility We've fix it only for the latest content type.

Change-Id: Ia4be8b384f8b7f193f139f2206ceea25786b5cc3
Closes-bug: 1250158
DocImpact
Partial-bug: #1277104
Merge "Fix glance V2 incorrectly implements JSON Patch'add'"