Update OS serverprovider schema to allow specifying image by uuid only

Existing images should not require URL, format etc, they should be
referenceable by uuid only. This patch adds an OR into the schema to
allow uuid for full image specification.

Change-Id: I6d96b43e5299706bdbd879939e94b3ca79feb810
Closes-Bug: #1278507
Merge "Update OS serverprovider schema to allow specifying image by uuid only"