Add missing tests for nova.db.api.network_*.

Added new tests for nova.db.api.network_*.
Moved existent tests for nova.db.api.network_* into NetworkTestCase.
Removed unused and invalid method network_get_all_by_instance().

blueprint test-db-api

Change-Id: Ib06135138d846f50f1aad1277afadf7639522226
Merge "Add missing tests for nova.db.api.network_*."