[5705] kea-dhcp4 now fully supports globl HRs

[5705] kea-dhcp4 now fully supports globl HRs

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
    Added support for HR_GLOBAL mode

src/bin/dhcp4/tests/dhcp4_test_utils.cc
    Minor modifications to output exception and config text
    when parsing fails.

src/bin/dhcp4/tests/host_unittest.cc
    New tests to make sure reservation matching behaves
    according to reservation modes

src/lib/dhcpsrv/network.cc
src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    Adding missed bits needed to parse HR_GLOBAL mode