CFINSPEC-292 Add support for waivers

[clintoncwolfe]

Description

Waiver file support was claimed, but in fact had not been implemented. This implements it, renaming the config key to be consistent, and adds a test case.

Issues Resolved

Fixes #290

Check List

• [x] New functionality includes tests
• [x] All tests pass
• [x] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco
• [x] PR title is a worthy inclusion in the CHANGELOG