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