Set is_public to False by default for volume backed snapshots

This update results in the same behavior that is already implemented
in _create_image where the meta_data for is_public is set to False.

Change-Id: Ie0720d5c847871527db7d9477ca6cb05abe7a1a5
Closes-Bug: #1255316
Merge "Set is_public to False by default for volume backed snapshots"