Clarify about how BES uses os_flavor

We don't. We don't use os_flavor anymore because it is no longer implemented, therefore the mixin should not provide that information as a usable key in REQUIREMENT_KEY_SET. We get the information from os_name instead.

**Verification**

- [x] Make sure Travis is green
- [ ] Try testing a BES-powered exploit. You should either see it trying to serve the exploit, or it should at least flag the unmet requirements correctly.