Description of problem:
If you have an already issued token and want to use it for a "nova list" command, for example, you need to run the following command **without** sourcing stackrc or overcloudrc:

nova --os-auth-token $OS_TOKEN --os-auth-url $OS_AUTH_URL list

If you try, then you get this error:
ERROR (TypeError): __init__() got an unexpected keyword argument 'username'


Version-Release number of selected component (if applicable):
python-novaclient-2.23.0-2.el7ost.noarch


How reproducible:
100%


Steps to Reproduce:
1. Don't source any rc file
2. Get a keystone token (a sample bash script to get a token is attached to the bug - you just need to edit the first few lines in it according to your environment)
3. Run the following command (replace the params with the real values): nova --os-auth-token $OS_TOKEN --os-auth-url $OS_AUTH_URL list


Actual results:
ERROR (TypeError): __init__() got an unexpected keyword argument 'username'


Expected results:
The nova command should support using an already-issued token