[master] Merged trac3927 (default in option-def)

diff --cc doc/guide/dhcp4-srv.xml
Simple merge

diff --cc doc/guide/dhcp6-srv.xml
Simple merge

diff --cc src/bin/dhcp4/json_config_parser.cc
Simple merge

diff --cc src/bin/dhcp4/tests/config_parser_unittest.cc
Simple merge

diff --cc src/bin/dhcp4/tests/fqdn_unittest.cc
Simple merge

diff --cc src/bin/dhcp6/tests/config_parser_unittest.cc
Simple merge

diff --cc src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
Simple merge