Add support for best available QoS

[mxgrey]

While investigating #554 I noticed that we are currently missing options for users to set "best available" QoS for various settings. The "best available" option was introduced in Iron, so I have suppressed the new feature when compiling for humble.

I also added a unit test to validate that rcl is seeing the correct QoS as set by the user. I have not been able to corroborate the report in #554.. it seems QoS is working just fine.