Make auth plugins dest save to os_

If the auth plugin saves into the normal namespace like .user_id and
user_id is an argument of the command then the two argument collide with
each other.

This is fairly common, particularly in keystoneclient's shell.

There is a little bit of a compatibility concern in that the variables
on the returned namespace have changed, however the usage of this
function should be if you use register_argparse_arguments you should
also use load_from_argparse_arguments and that is not changed.

Change-Id: Id1cb0983a1e78661492acd78ad9aa67ff8d49250
Merge "Make auth plugins dest save to os_"