[#413,!288] kea-dhcp6 now uses options from config backends

[#413,!288] kea-dhcp6 now uses options from config backends

src/bin/dhcp6/tests/config_backend_unittest.cc
    TEST_F(Dhcp6CBTest, mergeOptions) - enabled and
    revamped.

src/lib/dhcpsrv/tests/cfg_option_unittest.cc
    TEST_F(CfgOptionTest, createDescriptorOptionValid) - added test
    of a standard V6 option

src/lib/dhcpsrv/srv_config.cc
    SrvConfig::merge6(SrvConfig& other) - now merges
    options