Fix type mismatch errors in NetworkTestCase

A few tests of NetworkTestCase use string literals as
network_id values. This works great when running tests
on both SQLite and MySQL, but fails on PostgreSQL, which
is very string when checking data types.

Blueprint: db-api-tests-on-all-backends

Change-Id: I090e42ff045379ac404e7bc5c91fbc82a52920b0
Merge "Fix type mismatch errors in NetworkTestCase"