If no password in env or command line, try prompting.
If no password in env or command line, try prompting.

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

hughsaunders/python-openstackclient,metacloud/python-openstackclient,varunarya10/python-openstackclient,dtroyer/python-openstackclient,derekchiang/python-openstackclient,metacloud/python-openstackclient,dtroyer/python-openstackclient,derekchiang/python-openstackclient,varunarya10/python-openstackclient,hughsaunders/python-openstackclient,openstack/python-openstackclient,redhat-openstack/python-openstackclient,citrix-openstack-build/python-openstackclient,BjoernT/python-openstackclient,BjoernT/python-openstackclient,citrix-openstack-build/python-openstackclient,openstack/python-openstackclient,redhat-openstack/python-openstackclient