Add negative tests for network

Create a base class for negative and positive network tests
Add some negative tests for network

- test_update_non_existent_network
- test_delete_non_existent_network

Blueprint negative-test-files

Change-Id: Ib7df4f4ee40a0b4bf43bffa8fe123c321b0b2d9e
Merge "Add negative tests for network"