[tests/vxlan]: Fixed VxLAN default parameters and scale limits

[Aakash Tiwari (aakashtiwari2630-max)]

Porting master PR: sonic-net/sonic-mgmt#25063

Summary of fix:

1. Currently the defaults set in conftest were pretty high and readjusting the defaults in the test doesn't work since default is already set. So, the correct approach is to fix the defaults in conftest.
2. The ECMP endpoint tests currently doesn't check for maximum scale limits. This has problem since configuring higher limit will fail the test and subsequent tests. Added logic to check for platform limits.
3. Currently there is no cleanup of vnet route after each test which could have issues in case we run the whole suite and a particular test fails. Added clean_vnet_route which has scope function so that it runs after each test and every test runs with a clean slate.
4. Currently, there is no logic to reconfigure vxlan router mac after config reload. Due to this, tests will fail. Added APIs to reprogram router mac after reload.
5. In VxLAN scale tests, currently the sleep is fixed as 20s. This should be dependent upon the route scale. Added scale aware logic in sleep.

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