Make source URL configurable

[bikramj]

Description

We are restricting outgoing internet access to our machines and the community cookbook is not able to go to assets.nagios.com to grab source code. Want the source URL configurable so that we can download it from our local Nexus proxy repo

Issues Resolved

List any existing issues this PR resolves

Check List

• [ ] A summary of changes made is included in the CHANGELOG under ## Unreleased
• [ ] New functionality includes testing.
• [x] New functionality has been documented in the README if applicable.