Show private key in response if user ask to regenerate it

Hi folks,

Chef API description about how to update an user with the PUT method includes this comment:

```PUT accepts a boolean: { "private_key": "true" }. If this is specified, a new private key is generated.```

However, knife-opc is not showing the full server response and you can't see the new private key when is regenerated.

I don't know if my PR is acceptable but at least is a step in the right direction, IMO.

Thanks for open sourcing this, btw.

Miguel,
I'm wondering if we should suppress this "Saved #{user_name}" message by default to make it easier to use this command in a pipeline or with file handle redirection when a key is returned:

    knife opc user edit USERNAME > foo.pem