src/bin/dhcp6/json_config_parser.cc
        - createGlobalDhcp6ConfigParser() - creates ClientClassDefListParser
        for "client-classes" element
        - configureDhcp6Server() - runs the ClientClassDefListParser against
        the "client-classes" element

    src/bin/dhcp6/tests/config_parser_unittest.cc
        Added new tests for parsing client class definitions:
        TEST_F(Dhcp6ParserTest, clientClassDictionary)
        TEST_F(Dhcp6ParserTest, invalidClientClassDictionary)

    src/bin/dhcp6/dhcp6.spec
        Added global "client-classes" element