[5535] kea-dhcp6 parsing now supports relay "ip-addresses"

[5535] kea-dhcp6 parsing now supports relay "ip-addresses"

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
    Modified to support "ip-address" and
    "ip-addresses" under "relay"

src/bin/dhcp6/tests/get_config_unittest.cc
    Updated per process.

src/bin/dhcp6/tests/config_parser_unittest.cc
    void checkResult() - modified to output parsing error text

    TEST_F(Dhcp6ParserTest, subnetRelayInfo)
    TEST_F(Dhcp6ParserTest, sharedNetworksDerive)
    - updated to use new Network methods

    TEST_F(Dhcp6ParserTest, subnetRelayInfoList) - new test
    to verify a list of addresses in relay