Remove keystone-manage import_nova_auth

import_nova_auth has been dropped as part of change
If7277e912f11c9bf3bec15f9addd848e0774f14f. Remove it from the
documentation as well.

Change-Id: Ib213d1937a70ddd88a22009d168f924d9adeceae
Closes-Bug: #1202354
Merge "Remove keystone-manage import_nova_auth"