Addressing more review comments...

new file - host_unittest.cc
    contains Dhcp6Client based host reservation tests

dhcp6_client.cc
dhcp6_client.h
    added setDuid(str) method to allow duid to be set to
    specific value

dhcp6_srv_unittest.cc -
    deleted host reservation tests, now in host_unittest.cc

dhcp6_test_utils.cc
dhcp6_test_utils.h
    deleted createHost6() method