[#35,!517] Rework kea-dhcp4 parsing and server

[#35,!517] Rework kea-dhcp4 parsing and server

src/bin/dhcp4/ctrl_dhcp4_srv.cc
    Removed setting D2ClientCfg fetch function

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_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/dhcp4/dhcp4_srv.cc

    Dhcpv4Srv::processClientFqdnOption()
    Dhcpv4Srv::processHostnameOption()
    Dhcpv4Srv::assignLease()
        Use scoped DdnsParams from context to make decisions, pass
        into D2ClientMgr functions, and supply hostname sanitizer

src/bin/dhcp4/json_config_parser.cc
    configureDhcp4Server()
    - use SrvConfig::moveDdnsParams() to move dhcp-ddns parameters
    - handle new "ddns-" globals

src/bin/dhcp4/tests/config_parser_unittest.cc
src/bin/dhcp4/tests/d2_unittest.cc
src/bin/dhcp4/tests/dhcp4_test_utils.cc
src/bin/dhcp4/tests/fqdn_unittest.cc
src/bin/dhcp4/tests/get_config_unittest.cc
    Updated and new tests