Remove username and password from configuration.

It is possible that we will want to re-enable username/password
authentication, but this will require further code changes anyway so
removing for now.
Switch to using client_credentials strategy.

This strategy allows you to authenticate with just the client_id and
client_password. However, it normally limits access to a basic level -
nothing that is per-user.

The Zoning API has been configured to allow full access to 'trusted'
applications without authentication. This means that for trusted
applications, the client_credentials strategy gives the same level of
access as the other strategies.