cbbackup/cbrestore should accept unechoed password in the command prompt

If the user is specified but not the password, we should allow the user to input the password unechoed at the command prompt.
This is frequently desirable because admins often work in teams or with Couchbase on screen shares to collaborate when working on a cluster. It's very desirable for the admin operating the terminal to copy the password from some password management app to the terminal prompt using the clipboard and in this way not share the password with all the other admins on the screen share.
In python it's super easy:
import getpass
if args.user and not args.password
args.password = getpass.getpass()