Delegation of network and ports management to dvs-agent

Setting as incomplete because I don't now right awser to a question:
How mech_driver communicates to agent that network/port should be created/updated/deleted. Taking to an assumption that mech driver inherits from SimpleAgentMechanismDriverBase

It looks like that, if we want delegate ports/networks management to agent, we would have to implement everything by hand.
This isn't something that ovs mech driver does either.

Furthermore decision to move this functionality to agent should depend on vSphere API. if it can deal with concurrent requests
then we may leave management in the plugin, if it should be serialized - then we need to put it into the agent.

In our case, port and networks management works fine as it is now implemented, so without other argument it should stay in driver.

vSphere does not support concurrent access for change resources. And does not support queues for it.
OpenStack can manage vSphere resources by minimal clusters blocks over special compute nodes with vSphere API. It can manage multiple vSpheres because each compute node has its own configuration.

Agent may help us to solve multiple problems:

1. RSG requests serialization
2. Scailing by dividing resources to pools (clusters), no bottleneck
3. Multiple vSphere support (at the future)
4. Multiple physnet support
5. Reducing the load on the controller
...

Ok, so it looks, that we can't rely on AgentNotifierApi class to pass all requests to agent.

Each request should be passed by corresponding driver method (update_port_postcommit, create_network_precommit, etc.) if necessary.