When action is 'append', let's check value by value.
Previously, we used `convert_value' in all cases, which seems
to try to detect lists based on the value of nargs, but that fails
with lists of one item. This "worked" for 'str' options, because
the values don't get checked, but was failing for choice types
(see Issue #772), and would fail for other types like int.
unit tests for pip.baseparser.ConfigOptionParser