[Thinkit] Use a new test fixture for sflow reboot test. Collect ASIC_DB and CONFIG_DB in sFlow debug logs.Add an API to set sflow interface config. Verify the packets received at SUT when sending traffic from control switch. Change backoff test traffic speed to generate 850 samples/sec

[Thinkit] Use a new test fixture for sflow reboot test. Collect ASIC_DB and CONFIG_DB in sFlow debug logs.Add an API to set sflow interface config. Verify the packets received at SUT when sending traffic from control switch. Change backoff test traffic speed to generate 850 samples/sec #4225