Adds negative tests to test_servers.py

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

Following are the negative tests -

        1. test_create_server_with_invalid_name
        2. test_get_server_with_invalid_id
        3. test_delete_server_with_invalid_id

Change-Id: I2a736be7c18290f7fcad65541d33dad9216e213f