Support for keystone auth plugins

This patch allows the user to choose which authentication plugin
to use with the CLI. The arguments needed by the auth plugins are
automatically added to the argument parser. Some examples with
the currently available authentication plugins::

  OS_USERNAME=admin OS_PROJECT_NAME=admin OS_AUTH_URL=http://keystone:5000/v2.0 \
  OS_PASSWORD=admin openstack user list

  OS_USERNAME=admin OS_PROJECT_DOMAIN_NAME=default OS_USER_DOMAIN_NAME=default \
  OS_PROJECT_NAME=admin OS_AUTH_URL=http://keystone:5000/v3 OS_PASSWORD=admin \
  OS_IDENTITY_API_VERSION=3 OS_AUTH_PLUGIN=v3password openstack project list

  OS_TOKEN=1234 OS_URL=http://service_url:35357/v2.0 \
  OS_IDENTITY_API_VERSION=2.0 openstack user list

The --os-auth-plugin option can be omitted; if so the CLI will attempt to
guess which plugin to use from the other options.

Change-Id: I330c20ddb8d96b3a4287c68b57c36c4a0f869669
Co-Authored-By: Florent Flament <florent.flament-ext@cloudwatt.com>