[#3770] v4 global options work

[#3770] v4 global options work

Fixed fetch of global options added UT tests

/src/hooks/dhcp/mysql/mysql_cb_impl.cc
    MySqlConfigBackendImpl::getOptions() - take client-classes into account
    MySqlConfigBackendImpl::createInputClientClassesBinding() - use ClientClasses::toElement()

/src/hooks/dhcp/mysql/tests/mysql_cb_dhcp4_unittest.cc
    TEST_F(MySqlConfigBackendDHCPv4Test, globalOption4WithClientClassesTest)
    TEST_F(MySqlConfigBackendDHCPv4Test, getAllOptions4WithClientClassesTest)
    - new tests

/src/hooks/dhcp/pgsql/pgsql_cb_impl.cc
    PgSqlConfigBackendImpl::getOptions() - take client-classes into account

/src/hooks/dhcp/pgsql/tests/pgsql_cb_dhcp4_unittest.cc
    TEST_F(PgSqlConfigBackendDHCPv4Test, globalOption4WithClientClassesTest)
    TEST_F(PgSqlConfigBackendDHCPv4Test, getAllOptions4WithClientClassesTest)
    - new tests

/src/lib/dhcpsrv/parsers/option_data_parser.cc
    OptionDataParser::createOption() - use ClientClasses::fromElement()

/src/lib/dhcpsrv/testutils/generic_cb_dhcp4_unittest.*
    GenericConfigBackendDHCPv4Test::TearDown() - skip schema destroy
    if env variable KEA_UNIT_TEST_KEEP_SCHEMA is defined

    GenericConfigBackendDHCPv4Test::makeClassTaggedOptions()
    GenericConfigBackendDHCPv4Test::updateClassTaggedOptions()
    GenericConfigBackendDHCPv4Test::globalOption4WithClientClassesTest()
    GenericConfigBackendDHCPv4Test::getAllOptions4WithClientClassesTest()
    - new tests