Use options.os_auth_url rather then environment variable

If you do specify --os_auth_url rather than using the environment variable
OS_AUTH_URL, you'll end up with different behaviour.

Change-Id: I7430c2a619487375427b1c377ce00245eff107ff

bin/swift

@@ -1865,7 +1865,7 @@ def parse_args(parser, args, enforce_requires=True):
     (options, args) = parser.parse_args(args)
 
     if (not (options.auth and options.user and options.key) or
-            environ.get('OS_AUTH_URL')):
+        options.os_auth_url):
         # Use 2.0 auth if none of the old args are present
         options.auth_version = "2.0"