Use correct device_manager member in dhcp driver

* The DHCP driver used to use device_delegate, but that has
  changed to device_manager. referencing device_delegate
  caused the cleanup of the dhcp agent namespaces to fail.
  This fixes the issue by using the correct member name,
  device_manager

Change-Id: Iab8ea0549383335ff66f3f13df1953d206eeb28f
Closes-Bug: #1245408
Merge "Use correct device_manager member in dhcp driver"