Actually test interactive password prompt

* keystoneclient/tests/v2_0/test_shell.py
(ShellTests.test_user_create_password_prompt): Remove the password
from the calling environment and fake out the check for an
interactive terminal session so that the code which implements
password prompting will actually be exercised.

Change-Id: I46f45553995316c7d006a83897413d57e127e48c
Merge "Actually test interactive password prompt"