nec plugin: Avoid long transaction in delete_ports

nec plugin: Avoid long transaction in delete_ports

db_plugin.delete_ports() can lead to long transaction
if plugin.deleete_port talks with external system.
This commit removes a transaction in delete_ports and
allows NEC plugin to use more granular db transactions
in delete_port. It greatly helps db race conditions and
timeouts in delete_port operations.

To avoid to impact other plugins/drivers by changing
db_plugin.delete_ports directly and to land this patch soon,
this commit overrides delete_ports() in NEC plugin.
Further disssion on transaction in delete_ports will be
discussed under bug 1282925.

Closes-Bug: #1282922
Related-Bug: #1282925

Change-Id: I2c00694ad34eb2058bf7a0ff1c920ceded327d43