Increase limit of managed policies from 10 to 20

## Problem

Wonk is hard-coded to allow up to max 10 policies per role. However, the limit in AWS can be increased up to 20 (hard limit). [Source](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_iam-quotas.html#reference_iam-quotas-entities)

## Potential solutions
- change limit from 10 to 20
- make it possible to override limit via a cli flag