Protects for cases where flavor already exists or already deleted in nov...

...a

since we are all pointing at the same nova setup  (with same creds/tenant right now) we are hitting sync cases with duplicate flavor creation, or trying to delete a flavor that someone else has already deleted. This puts some simple checks in place and prevents exposions.

For example, see  https://github.com/tuskar/tuskar/issues/64

thanks, marios