cb-cmds: inheritance in config file should be overridable in config-backend
When in config file there is defined a subnet with unspecified reservation-mode then changing reservation-mode in global scope will not affect this predefined subnet. But it is expected that parameters from global scope are inherited by subnet.
test case in forge:
tests/dhcpv4/kea_only/config_backend/test_reservations.py::test_reservation_mode_in_globals[v4]