Fix `chef-server-ctl password` command.

Uses knife opc instead of blown away code.

Now accepts `--enable-external-auth` in addition to `--disable` and updates output to display `--enable-external-auth` in place of `--disable`. Will also update the docs. This is a much more descriptive and intuitive name for new users but we want to leave `--disable` in for old users.

Depends on https://github.com/opscode/knife-opc/pull/20.

[Build](http://wilson.ci.opscode.us/job/chef-server-12-trigger-ad_hoc/130/downstreambuildview/)