[fix] replaced retrieval of client with search for user

The ClientManagement endpoint has been moved from resource to auth
server and is not currently available. Therefore we switched to user
searching to test correct authentication via client credentials flow.
[fix] removed 'de' from Accept-Language header

This is necessary to work around a bug in the current auth-server that
fails content negotiation, if the first locale in the Accept-Language
header is not known.
[enhancement] bringing back wire and header logging for http client