Acceptance Tests: make username/password configurable

Acceptance Tests: make username/password configurable

This makes the username/password used for authentication configurable,
because some guest operating systems may have restrictions on accounts
to be used for logins, and it just makes it better documented.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210412044644.55083-7-crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>