add required ext for subnetpool tests

tests with required default-subnetpool extencion:
  test_create_subnetpool_with_timestamp
  test_update_subnetpool_with_timestamp
  test_show_subnetpool_attribute_with_timestamp

Related-prod: PRODX-3721
Change-Id: I31882a8cd25d2ae229d63bf872b729e7a36b6798
(cherry picked from commit 676340a8ba551b07d68c27fb51ba58a8a302f2b6)
(cherry picked from commit 319bd38876e2d4c4d219b602c93395362051b2ee)
1 file changed