src/bin/dhcp4/json_config_parser.cc
    - createGlobalDhcp4ConfigParser() - creates ClientClassDefListParser
    for "client-classes" element
    - configureDhcp4Server() - runs the ClientClassDefListParser against
    the "client-classes" element

src/bin/dhcp4/tests/config_parser_unittest.cc
    Added new tests for parsing client class definitions:
    TEST_F(Dhcp4ParserTest, clientClassDictionary)
    TEST_F(Dhcp4ParserTest, invalidClientClassDictionary)

src/bin/dhcp4/dhcp4.spec
    Added global "client-classes" element