NMIP4Config, NMIP6Config, and NMPlatform shall share one
NMDedupMultiIndex instance.

For that, pass an NMDedupMultiIndex instance to NMPlatform and NMNetns.
NMNetns than passes it on to NMDevice, NMDhcpClient, NMIP4Config and NMIP6Config.
So currently NMNetns is the access point to the shared NMDedupMultiIndex
instance, and it gets it from it's NMPlatform instance.

The NMDedupMultiIndex instance is really a singleton, we don't want
multiple instances of it. However, for testing, instead of adding a
singleton instance, pass the instance explicitly around.