Interactive prompt for create user

Execution of the shell will now prompt the user for a password if argument
'--pass' is specified without a following parameter.  In that way, a user does
not need to pass passwords on the command line.

Usage example:
$ keystone user-create --name bob --tenant admin --pass --enabled true
New Password:
Repeat New Password:

Closes-Bug: #1100116
Change-Id: I1f6d6322830972dfad19ebe2fe63e91f82ed8033
Merge "Interactive prompt for create user"