NEC plugin: Fix nec-agent unexpected exit due to RPC exception

Changes nec-agent to catch RPC exceptions in the main
daemon loop. It fixes the following two bugs.

- nec-agent dies with RPC timeout in secgroup RPC
  (Closes-Bug: #1235106)

- port_added message can be dropped when RPC timeout occurs
  (Closes-Bug: #1235111). Moves exception catch from PluginApi
  to the main daemon loop.

Change-Id: I12394b739fc35bff271543f4625c5e3152e534ae
Change rpc_support_old_agents default to False

Changes the rpc_support_old_agents config variable default value to
False, so that the vlan_id field (redundant with the segmentation_id
field) is no longer included in RPC messages from the linuxbridge and
mlnx plugins to their corresponding L2 agents. This can be set to True
if needed to support L2 agents older than midway through the havana
cycle.

Closes-Bug: 1235323
Change-Id: I919824c5b02e28e3563e64e98bb106bf722327ec
Update ML2 README file for havana

Closes-Bug: 1235308
Change-Id: I684581c00ff28bc9cdb36034a9a970275b0a0090
Merge "Update ML2 README file for havana"
Merge "NEC plugin: Fix nec-agent unexpected exit due to RPC exception"
Merge "Change rpc_support_old_agents default to False"