[#1672] class fixed field order now matches class option order in kea-dhc4

[#1672]  class fixed field order now matches class option order in kea-dhc4

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::setFixedFields - modified to use the value for a field
    from the first class in query's list of classes that specifies
    the field.

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
    TEST_F(Dhcpv4SrvTest, fixedFieldsInClassOrder) - new unit test