Add more tests for positional CLI opts

Add tests to ensure that e.g. default=None, default=[], etc. works
as expected.

Change-Id: I66f5df6e622a66faa1d00ee0846cbec5eb119283
Add warning about interpolating values from groups

Add a warning explaining that options in groups cannot be used with
the interpolation syntax.

Reformat note to use directive so it is highlighted.

Add "docs" section to tox.ini to make testing doc build easier.

Change-Id: If5106aa44d4c87abe84a7fb7a250541bba9f5068
Updated from global requirements

Change-Id: I4f210812d4b2df3992b8444f384839b36f807123
Merge "Add more tests for positional CLI opts"
Merge "Add warning about interpolating values from groups"