Support configure get/set with nested attrs

This only works with the full qualified name, i.e.
with the profile specified.  For example:

    # Works:
    aws configure set default.s3.signature_version s3v4
    aws configure get default.s3.signature_version s3v4
    aws configure set profile.prod.s3.signature_version s3v4
    aws configure get profile.prod.s3.signature_version s3v4

    # Does not work
    aws configure set s3.signature_version s3v4
    aws configure get s3.signature_version s3v4
May need to rethink the approach.  I've left a failing test
to make it easy to pick right up.

cc @danielgtaylor