Commit 031609b572 changed the way
is_public was munged. However, when testing it it looks like it
was always seen as an empty string after this, so all created
images were private. Changing the way the 'if' block is done fixes
it.
Change-Id: If2eb52a6d0d9be89c9323702616f62f8ecba5e02
06182be5f7