sw test tool: Remove an unnecessary wrapper method

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Reduce similar classes

TargetSw and TesterSw are merged as OpenFlowSw.

And to avoid pylint error, add dummy methods to DummyDatapath.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Reduce similar methods of handling events

All of xxx_stats_reply_handler() are merged as stats_reply_handler().

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Reduce similar methods of deleting flows

OpenFlowSw.del_test_flow() and OpenFlowSw.del_flows_for_throughput_analysis() are merged as OpenFlowSw.del_flows().

And rename OpenFlowSw.del_test_meter() to OpenFlowSw.del_meters(), like OpenFlowSw.del_flows().

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Reduce similar methods of initializing flows

_test_initialize_flow_tester() is merged into _test_initialize_flow().

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
sw test tool: Reduce similar methods of checking entries exist

All of _test_xxx_exist_chk() are merged as _test_exist_chk().

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>