Cleanups for create-flavor

Instead of setting defaults values to None in the signature and then setting
the *real* default in the function body, it's more concise/readable to set the
value in the signature itself (when dealing with immutable types).

Along with this, the handling of `memory_mb` and `vcpus` was detangled so the
attributes were handled in one place and not across both the non-negative and
positive int checks.

Change-Id: I1c06768c2fd076adc8c0a647a492dabb67b7bf79
Merge "Cleanups for create-flavor"