IP Neighbor: RTNETLINK answers: File exists, Failed to add static arp entries

[fkwilken]

• When consecutively running tests that use ip neighbor or IpNeighbor.add, many fail with the error RTNETLINK answers: File exists, leading to AssertionError: Failed to add static arp entries
• This seems to happen when an identical configuration is added, suggesting that previous configuration has not been cleaned up.
• Either IP Neighbor cleanup is not working properly, or 'add' calls could be changed out for 'replace', which overrides previous config without failing.

Tests affected:

test_ipv4_replace_dyn_stat_arp
test_ipv4_static_route_over_static_arp
test_ecmp_nexthops_down[3]
test_ecmp_traffic_distribution[basic]
test_ecmp_distribution_lags
test_hw_drop_l3[3]