Use Tempest-lib's compute networks_client

Compute networks_client has been migrated to
tempest-lib- I3d14142e78f72876e4cdc6ce3d3c7cd210b3c670

Now Tempest-lib provides this client as stable interface so Tempest
should start using this from lib and remove its own copy.
Also remove its unit tests.

This commit makes Tempest to use compute networks_client
from Tempest-lib.

Change-Id: Icb48608b64d2cac5b4a41b2b79d55b2c24517e67
4 files changed