Put a cap on our cyclomatic complexity

flake8 has support for cyclomatic complexity (Mccabe) currently our
worst offender has a complexity of 18 (25 is considered very bad). So
set our max-complexity to 20 so we keep our complexity in a great shape.

https://github.com/flintwork/mccabe

Change-Id: I682f3c192e46bd50236b10a337f0b129c4b02eb8