There are severail problems in TestShowNetwork:
1. NETWORK_ITEM is not used.
2. Some redundant code.
3. The param n_mock is used to fake find_attr(), but the name is confusing.
So rename it to find_attr().
This patch fixes them.
Change-Id: I7bc909057125013fb9d215e92c61f847300f93e2
Implements: blueprint osc-network-unit-test-refactor
Related-to: blueprint neutron-client