Use a copy of NEW_NETWORK for test_networks

In test_networks, NEW_NETWORK is used as a request body between unit
tests. "Create a network" API changes a request body internally, so
each test should use a copy of NEW_NETWORK to avoid affecting to the
other unit tests. However one test uses NEW_NETWORK directly.
This patch makes each test use a copy of NEW_NETWORK for avoiding
similar problem in the future.

Change-Id: I9e12fe89956d18dde54bec83c15af258d66e0682
Merge "Use a copy of NEW_NETWORK for test_networks"