Implements blueprint password-prompt
This logic was recently added to keystone as a short term fix. The long
term fix is to have that same logic here. Basically, if no password is
present in the env or command line and there's a tty available for us
to prompt, then attempt to use getpass.
Change-Id: Ia0eec800b96c8f6ca1c2540e21e0e03c3880c713
39da32b3a5