Fix is_public munge

Commit 031609b57284cf13877ce727c2e4672831a3dbe2 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
Merge "Fix is_public munge"