If Zuul's identity provider supports Direct Access Grants, the
user can provide their credentials to zuul-client instead of
an auth token. The credentials can be passed as the --username
and --password CLI arguments, or as fields in the configuration file.
If no password is provided, zuul-client will prompt the user to
provide one.
Change-Id: Ief1a1e8d8f763239357d926dd10407a4ed5d8f37
ced7a372a7