Implements keystone portion of blueprint prompt-for-password
For security reasons, having the password on the command line
or in the environment is an issue for us. (See the blueprint for
details.) This change will simply make one attempt to prompt for
a password if (1) nothing was specified already and (2) there's
a tty available for the user to respond on. If we don't get a
password, then the existing error will be raised.
Remoted getpass from pip-requires, it's in the std lib
Tweaked a comment
Tweaked error message
Don't catch Ctl-C
Fix import to match conventions
Missed a tweak during the rebase.
Added suggested password prompt
Change-Id: I54bca2397da7bd366f7ac503e767b109efc093e7
e77234bd3e