l2-population/lb/vxlan : ip neigh add command failed

we were using ip neigh add command which must be replaced by
ip neigh replace, to avoid error when creating a VM with an ip
previously used by a deleted VM

Change-Id: I2405096d5925ae37efd5f8abcc02b99cf0c9f5d3
Closes-Bug: #1282662
Merge "l2-population/lb/vxlan : ip neigh add command failed"