Adds negative tests to test_records.py

This submission adds three negative tests under
"designate/tests/test_api/test_v1/test_records.py" script.

Following are the negative tests -
1. test_get_record_missing
2. test_get_record_with_invalid_id
3. test_delete_record_invalid_id

Change-Id: I06c1af89ba71e05d1a4da4b2ba1f0a41aa686c8c