Extending delete_batch function and related test

in models.py (cue/db)
* added delete_batch methods in Cluster and Node class to
  override delete_batch in base class.

in test_models (cue/tests/db)
* removed test_delete_invalid_id since no changes take place
* updating status field in test dictionaries to use Status class
* updated batch_get test to include batch delete

Change-Id: Iae04a1cf6c5d71c08f37dc1aaca8b168364f5129
Merge "Extending delete_batch function and related test"