positive integer - fix the validator

https://github.com/cloudtools/troposphere/blob/master/troposphere/validators.py
zero is neither positive nor negative and we need to correct the validator.

Initiated a discussion and added more details at the forum.
https://groups.google.com/forum/#!topic/cloudtools-dev/1-5u2ZaVpbo

Read troposphere/validators.py and review the linked forum discussion for the additional details. Confirm the positive-integer validator treats zero as neither positive nor negative, then update the validator and its relevant coverage so that behavior is verified.

Written by the indexing model from the issue text.