sss_cache: initialize names member of sss_domain_info
    
    sss_tc_fqname() called by sss_get_domain_name() requires that the names
    member of the sss_domain_info struct is set to work properly. If the
    names struct is properly initialized in sss_domain_info the separate one
    in the tool context is not needed anymore.
    
    Related to https://fedorahosted.org/sssd/ticket/1741