DATA_PROVIDER: BE_REQ as string in log message
    
    Add be_req2str() for translation BE_REQ to string.
    So we will have
    || Got request for [0x1001][FAST BE_REQ_USER][1][name=celestian]
    instead of
    || Got request for [0x1001][1][name=celestian]
    
    Function be_req2str() is used in data provider and in responder too.
    So this patch create new header file data_provider_req.h which
    delivers function be_req2str() and definitions of BE_REQ_*.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2708
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>