Add --auth_url, --username and --password and OS_* env vars as
aliases to --auth, --user and --key.
Default to --auth-version=2.0 if none of --auth, --user or --key
are set or if OS_AUTH_URL is set.
Ensure trailing '/' is present in --auth so URLs are created correctly.
Fixes lp925212
Change-Id: Ic0008c5d5c1ab4fddbaab5d982ab60fed2c50019