Support HTTP Basic Authenticiation for httpok checks

We are checking an endpoint that we have basic authentication on with `httpok` and figured this might be useful for others as well:
- Accept a `username:password` string for basic authentication
- Fix settimeout for https (was raising NoneType has no attribute settimeout)
 (Same fix as https://github.com/maxnaude/superlance/commit/2bf7498603ee351c6f6baa6a45e4ccd554802030)
- Move headers creation outside of retry loop since the headers stay the same

Let me know if there are any tests that should be written for this.