[#225] Added ddsn-ttl-percent parameter

[#225] Added ddsn-ttl-percent parameter

kea-dhcp4/6 both now support ddns-ttl-percent
at the global, shared-network, and subnet scopes.
If specified it is the percentage of the lease
lifetime to send to D2.

Not yet supported by CB.

modified:
doc/examples/kea4/all-keys.json
doc/examples/kea6/all-keys.json
src/bin/dhcp4/dhcp4_lexer.cc
src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.cc
src/bin/dhcp4/dhcp4_parser.h
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/json_config_parser.cc
src/bin/dhcp4/location.hh
src/bin/dhcp6/dhcp6_lexer.cc
src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.cc
src/bin/dhcp6/dhcp6_parser.h
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/dhcp6_srv.cc
src/bin/dhcp6/json_config_parser.cc
src/bin/dhcp6/location.hh
src/bin/dhcp6/tests/fqdn_unittest.cc
src/lib/dhcpsrv/cfg_globals.cc
src/lib/dhcpsrv/cfg_globals.h
src/lib/dhcpsrv/dhcpsrv_messages.cc
src/lib/dhcpsrv/dhcpsrv_messages.h
src/lib/dhcpsrv/dhcpsrv_messages.mes
src/lib/dhcpsrv/ncr_generator.cc
src/lib/dhcpsrv/ncr_generator.h
src/lib/dhcpsrv/network.cc
src/lib/dhcpsrv/network.h
src/lib/dhcpsrv/parsers/base_network_parser.cc
src/lib/dhcpsrv/parsers/base_network_parser.h
src/lib/dhcpsrv/parsers/simple_parser4.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
src/lib/dhcpsrv/srv_config.cc
src/lib/dhcpsrv/srv_config.h
src/lib/dhcpsrv/tests/ncr_generator_unittest.cc
src/lib/dhcpsrv/tests/network_unittest.cc
src/lib/dhcpsrv/tests/srv_config_unittest.cc