Support neutron service without security groups enabled

Neutron raises a 404 error if security group extension is not enabled.
With ML2, it appends if you set in ML2 config file:

  [securitygroup]
     enable_security_group = False


Change-Id: I3237898aeb87db07cb65fe76319237f3f5a75643
Merge "Support neutron service without security groups enabled"