BUG 8876 - Fix to remove ELAN interface flows when external network deleted

BUG 8876 - Fix to remove ELAN interface flows when external network deleted

    - Problem: when deleting external network, the underlying ietf
      Interface might have been removed from the config DS prior
      to the call to delete the ELAN interface. As a result, the call
      interfaceManager.getInterfaceInfo returns null and the
      subsequent call to delete the ELAN interface fails to delete the
      flow entries for external network's ELAN interface.

    - Fix: invoke interfaceManager.getInterfaceInfoFromOperationalDataStore
      when delete External Network's ELAN interface

Change-Id: I764f3516d34a6d79aacff8dbedabb55219799ac9
Signed-off-by: Vinh Nguyen <vinh.nguyen@hcl.com>