Why now?
* argparse was introduced in Python 3.2 and back-ported to Python 2.7.
Until we dropped Python 2.6 support, we were stuck on optparse.
* keystoneauth.loading.cli provides register_argparse_arguments and
load_from_argparse_arguments helper methods. Now that we're moving
toward Keystone Session support, argparse seems required.
Closes-Bug: 1553030
Change-Id: I5139fb64a8631a3010680090fd04345f95c55c7b