[5033] - kea-dhcp6 now uses new D2ClientConfigParser

[5033] - kea-dhcp6 now uses new D2ClientConfigParser

    src/bin/dhcp6/json_config_parser.cc
        createGlobalDhcp6ConfigParser()
        - added clause to invoke new D2ClientConfigParser to
        set staging config
        - added clause to apply staged D2ClientConfig (formerly done
        by parser commit

    src/bin/dhcp6/parser_context.h
    src/bin/dhcp6/parser_context.cc
        aded PARSER_DHCP_DDNS context

    src/bin/dhcp4/simple_parser6.h
    src/bin/dhcp4/simple_parser6.cc
        defined SimpleParser6::D2_CLIENT_CONFIG_DEFAULTS
        SimpleParser6::setAllDefaults() - now sets defaults
        for D2ClientConfig

    src/bin/dhcp6/tests/d2_unittest.cc
    src/bin/dhcp6/tests/fqdn_unittest.cc
        Updated replace-name-mode values and tests
        (true/false no longer supported)

    doc/guide/dhcp6-srv.xml
        Updated, replace-client-name no longer accepts booleans