samples/zoap_server: Also listen on the unicast address

Some applications are not prepared for using multicast address and
specifying the interface, so it may be convenient for the user to be
able to use a unicast address when communicating with the
samples. This aligns this sample with most of the networking samples.

Change-Id: Ic97ea5a346a650751b6e2cbfefad25a3e700278c
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>