\Allow removal of properties using glance v2 api

This adds support to the glance client library for removing
properties from an image. Properties to be removed must be
explicitly listed. There is no equivalent of the 'purge_props'
functionality in v1.

Also beefed up testing around create since some of the
functionality is the same as update.

Fixes bug 1206472

Change-Id: I16f4c39cdfc8a0cd07bede600461b7a289fbb080