UDN services follow-up work

[ricky-rav]

What is this issue about?

As discussed internally, the following points should be addresses in the near future:

1. Refactor node tracker
A node tracker is launched for every services controller, so one per network. There are parts in the node tracker that are node-specific, parts that are network-specific: investigate whether we can optimize that so that we have one controller that works across all networks instead of n controllers responding to the same node events.

2. Have a controller that maps namespaces to networks
Avoid parsing the json of all nads when looking up the network name of a given namespace.

3. Test that a service correctly moves from one network to another one
#4555 (comment)

4. Adapt network_segementation_service.go to the framework introduced in network_segementation.go

[ricky-rav]

cc @tssurya

[tssurya]

thanks @ricky-rav !! this is awesome