Add authenticate method to oauth code

Forgot to add one last method when I initially checked in
the first set of code; the authenticate method will return an
actual keystone token that the user may now use.

Also, I added some changes to other methods because the client
has been updated.

Change-Id: Ie2707689e0df1fb1bc92177f932baf23fe1ca920
Remove api = apiName calls from each method

As discussed in https://review.openstack.org/#/c/36352/ for each
command, we were setting api = identity or volume... etc,
this was for an old way of calling commands that are is no longer
used.

Also removed openstackclient/common/command.py

Change-Id: I2705f35d343f2ae729dc22d6aed0b852b2f8ca19
Merge "Add authenticate method to oauth code"