Add the --prompt option for the CLI which will cause the user to be
prompted to enter a password. Any password otherwise specified by
--key, --os-password or an environment variable will be ignored.
The swift client will exit with a warning if the password cannot be
entered without its value being echoed.
Closes-Bug: #1357562
Change-Id: I513647eed460007617f129691069c6fb1bfe62d7
33ad9fd4cc