[#32,!23] - Added server-tag global to SrvConfig and kea-dhcp4/6

[#32,!23] - Added server-tag global to SrvConfig and kea-dhcp4/6

src/bin/dhcp4/dhcp4_lexer.ll
src/bin/dhcp4/dhcp4_parser.yy
src/bin/dhcp4/json_config_parser.cc
    added server-tag

src/bin/dhcp4/tests/config_parser_unittest.cc
    TEST_F(Dhcp4ParserTest, serverTag) - new test

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/json_config_parser.cc
    added server-tag

src/bin/dhcp6/tests/config_parser_unittest.cc
    TEST_F(Dhcp6ParserTest, serverTag)

src/lib/dhcpsrv/parsers/simple_parser4.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
    Added server-tag to global defaults

src/lib/dhcpsrv/srv_config.cc
    Added server_tag_ memger to SrvConfig

src/lib/dhcpsrv/tests/srv_config_unittest.cc
    TEST_F(SrvConfigTest, serverTag)
    TEST_F(SrvConfigTest, unparseConfigControlInfo6) - new tests