network tag UT: avoid using mix-in test class

This is a follow-up patch on https://review.openstack.org/#/c/461195/
There is a suggestion to keep test code more straight-forward
and avoid using mix-in to reduce the code complexity.
This commit moves all logic implemented in _test_tag.py
into individual network tests.

Change-Id: I0a9f8c6cd758db9035b0fd60ce4b9bfc791b6cbd

Author: amotoki