x509: avoid use of nss.data_to_hex
    
    Avoid use of the nss.data_to_hex function for formatting certificate
    fingerprints.  Add our own helper functions to format the
    fingerprints as hex (with colons).
    
    Part of: https://fedorahosted.org/freeipa/ticket/6398
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>