NVP plugin:fix delete sec group when backend is out of sync

If a security group does not exist on the NVP backend, an error
should not be raised on deletion of the security group.

This patch changes the plugin behavior by deleting the record
from the database and just logging that the security group
was not found on the NVP backend.

Closes-Bug: #1251422

Change-Id: Ib8adf7a830ff336655fd83ad4118cde641adf284
Merge "NVP plugin:fix delete sec group when backend is out of sync"