[#401,!254] kea-dhcp4 now merges in options from config backend

[#401,!254] kea-dhcp4 now merges in options from config backend

src/lib/dhcpsrv/cfg_option.*
    CfgOption::merge() - new function which merges a given set
    of option descriports into the existing set

    CfgOption::createDescriptorOption - new function that uses an
    option definition factory to create a descriptor's option instance

src/lib/dhcpsrv/tests/cfg_option_unittest.cc
    TEST_F(CfgOptionTest, validMerge)
    TEST_F(CfgOptionTest, invalidMerge) - new tests