README changes

- Change --hp-zone to --hp-region
- Update '-Z' flag to '-R' flag
- Change misleading example for knife hp server create. (While the
OpenStack REST API takes the username/password, it seems Fog does not)
- Update description of how to get credentials in HP Horizon UI to match
fog's documentation:
https://github.com/fog/fog/blob/master/lib/fog/hp/docs/connect.md