[5033] - migrated D2ClientConfigParser to SimpleParser, kea-dhcp4 uses it

[5033] - migrated D2ClientConfigParser to SimpleParser, kea-dhcp4 uses it

src/bin/dhcp4/json_config_parser.cc
    createGlobalDhcp4ConfigParser()
    - added clause to invoke new D2ClientConfigParser to
    set staging config
    - added clause to apply staged D2ClientConfig (formerly done
    by parser commit

src/bin/dhcp4/parser_context.h
src/bin/dhcp4/parser_context.cc
    aded PARSER_DHCP_DDNS context

src/bin/dhcp4/simple_parser4.h
src/bin/dhcp4/simple_parser4.cc
    defined SimpleParser4::D2_CLIENT_CONFIG_DEFAULTS
    SimpleParser4::setAllDefaults() - now sets defaults
    for D2ClientConfig

src/bin/dhcp4/tests/d2_unittest.cc
src/bin/dhcp4/tests/fqdn_unittest.cc
    Updated replace-name-mode values (true/false no longer supported)

src/lib/dhcpsrv/parsers/dhcp_parsers.h
src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    D2ClientConfig now derives from SimpleParser

src/lib/dhcpsrv/srv_config.h
src/lib/dhcpsrv/srv_config.cc
    Added a D2ClientConfigPtr member to SrvConfig.

src/lib/dhcpsrv/tests/dhcp_parsers_unittest.cc
    Updated tests to set D2 client config defaults

doc/guide/dhcp4-srv.xml
    Updated, replace-client-name no longer accepts booleans