[#35,!517] Rework kea-dhcp6 parsing and server

[#35,!517] Rework kea-dhcp6 parsing and server

src/bin/dhcp6/ctrl_dhcp6_srv.cc
    Removed setting D2ClientCfg fetch function

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
    ddns-send-updates
    ddns-override-no-update
    ddns-override-client-update
    ddns-replace-client-name
    ddns-generated-prefix
    ddns-qualifying-suffix
    - added to DHCP4,SHARED_NETWORK,SUBNET4

    hostname-char-set
    hostname-char-replacement
    - added to SHARED_NETWORK,SUBNET4

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::processClientFqdn()
    Dhcpv6Srv::createNameChangeRequests()
    Dhcpv6Srv::updateReservedFqdn()
    - use cxt.getDdnsParams()

src/bin/dhcp6/json_config_parser.cc
    configureDhcp6Server()
    - use SrvConfig::moveDdnsParams() to move dhcp-ddns parameters
    - handle new "ddns-" globals

src/bin/dhcp6/tests/config_parser_unittest.cc
src/bin/dhcp6/tests/fqdn_unittest.cc
src/bin/dhcp6/tests/get_config_unittest.cc
    Updated tests