Added command `knife opc user password`.

Command either updates the password for a user or, if --enable_external_auth is passed instead of a password, it will enable ldap for that user.
Agreed, but knife processes --args as ruby symbols, and `:enable-external-auth` isn't a valid ruby symbol and it breaks things (it drops --args directly into `config`, see [here](https://docs.getchef.com/plugin_knife_custom.html#options)).

Is there a way around that?