If password is defined in keyring, use it; otherwise, prompt for the
password.

* openstackclient/shell.py
  import keyring
  OpenStackClient.authenticate_user(): Get password from keyring,
  if it is defined; otherwise, prompt for the password. If user
  enter a password, store it in keyring.
  OpenStackClient.get_password_from_keyring(): New method to
  get password from keyring.
  OpenStackClient.set_password_in_keyring(): New method go set
  password in keyring.

* toos/pip-requires
  Define keyring as one of dependent.

Change-Id: I36d3a63054658c0ef0553d68b38fefbc236930ef

…stackclient