L2 agent RPC add new RPC calls

L2 agent RPC add new RPC calls

This patch introduces two new RPC calls:
* update_device_list
* get_devices_details_list_and_failed_devices

update_device_up and update_device_down are the RPC calls
used by the L2 agent to notify the plugin that
a device is up or down. These calls accept only one device.
update_device_list accepts a list of devices that will
be put up or down and returns a list of devices for which
the operation failed.

get_devices_details_list_and_failed_devices is a new RPC
call that returns a list of devices details and a list of
devices for which it was not possible to retrieve the
details.

Thanks to these new calls, the agent will be able to update
the status of several devices using one RPC call
and to retry the operation only for the devices that failed
instead of performing a full resync.
This will be implemented in another patch.

This patch introduces the new calls only for the OVS agent.
A following patch will take care of the other L2 agents.

DocImpact
Partially-Implements: blueprint restructure-l2-agent

Change-Id: Idb4e81f4399cf66025ec7edfcb78801c15e493af