Allow admins to change other people's passwords.
Tweaks following review.
help text tweak.
Amend the Args so it shows the optional username.
Merge pull request #984 from howbazaar/change-pwd-by-admin

Allow admins to change other people's passwords.

This is the last bit of the initial identity work.

This branch allows admins to change the passwords of other users.
This functionality already exists in the api, and we just needed to hook up the CLI bits.

When changing someone else's password, a JENV file is written out.