test_drop_counters assumes mellanox DUTs are always connected to onyx fanouts

[daall]

Description
The fanouthost fixture in test_drop_counters checks whether duthost has a mellanox asic, at which point it will attempt to initialize an onyx/mellanox fanout switch. This logic breaks down if we have a mellanox DUT connected to a fanout switch that is running SONiC or EOS.

Steps to reproduce the issue:

1. Run a test like test_equal_smac_dmac_drop against a mellanox DUT that is connected to a non-mellanox fanout.

Describe the results you received:
The test will fail during setup because the fanout isn't running the expected OS.

Describe the results you expected:
The correct type of fanout should be instantiated and/or the test should be skipped if the DUT isn't connected to a supported fanout switch.

Additional information you deem important:
IMO, now that the concept of a FanoutHost has been added in sonic-mgmt I think we should depend on that/let the framework figure out what type of fanout switch we have and the update/restore methods should be moved into there instead of writing this custom fanout fixture just for the drop counter tests.