[#365,!296] Added kea-dhcp6 parsing support for calculate-tee-time and percents

[#365,!296] Added kea-dhcp6 parsing support for calculate-tee-time and percents

src/bin/dhcp6/dhcp6_lexer.ll
src/bin/dhcp6/dhcp6_parser.yy
src/bin/dhcp6/json_config_parser.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
    added support for calculate-tee-times, t1-percent and t2-percent

src/bin/dhcp6/tests/tee_times_unittest.cc
    - new file with tests for t1/t2 determination

src/lib/dhcpsrv/parsers/dhcp_parsers.cc
    Subnet6ConfigParser::initSubnet() - added invocation
    of  parseTeePercents()

src/lib/dhcpsrv/tests/cfg_shared_networks6_unittest.cc
    updated tests

src/lib/dhcpsrv/tests/cfg_subnets6_unittest.cc
    TEST(CfgSubnets6Test, teeTimePercentValidation) - new test