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.

test_nec_agent.py is added during Havana.
The base class and related tests are imported.

Change-Id: I12394b739fc35bff271543f4625c5e3152e534ae
(cherry picked from commit 3df505e448d38929c20721b22d81e47bf917af6a)
Merge "NEC plugin: Fix nec-agent unexpected exit due to RPC exception" into stable/grizzly