Force the user to specify a flavor in the CLI

In the CLI a flavor is assumed even though a volume is not. So the only thing the CLI asks for when you type "reddwarf-cli instance create" is the --size parameter, which can be misconstrued as flavor size.

In a recent meeting, someone mentioned how this had tripped them up. Thinking on it more, using a default flavor doesn't really make sense- its too hard to figure out what that flavor will be, and the work it saves in typing isn't really important since the creation of an instance is relatively rare. It also is clearer if people have to specify a flavor.

Thus, we need to take away the default value in the CLI. If there's a default in the Instance create method in the client code itself that should go away too.