[#4585] Addressed more comments

Replaced getDnsName() with getFqdnLower()

modified:   src/bin/d2/d2_update_mgr.cc
modified:   src/bin/d2/d2_update_mgr.h
modified:   src/bin/d2/tests/d2_update_mgr_unittests.cc
modified:   src/lib/d2srv/nc_trans.cc
modified:   src/lib/d2srv/nc_trans.h
modified:   src/lib/dhcp_ddns/ncr_msg.h