[r=andreserl][bug=1557434][author=allenap] Backport r5173 from trunk: For the MAAS CLI, mimic the error behaviour provided by argparse 1.1 on PyPI.
This differs substantially from the argparse 1.1 found in the standard library. This branch also forces the development environment to use argparse as found in the standard library instead of the one pulled in by unittest2.