Fix leaky network tests

`test_networks.py:test_validate_instance_zone_for_dns_domain` was setting a
config in a way such that it could leak to other tests: using CONF.foo instead
of self.flags(foo=X).

This would cause other tests to break in non-determinstic ways, hence it
landing in trunk (order of tests running mattered!).

The test succeeds with the line removed, meaning it was probably only relevant
in an earlier version of the code, so the solution is to remove it.

Fixes bug 1205034

Change-Id: Iaa143a215e285d9b97c10d81bd9916ed3feeb04b
Merge "Fix leaky network tests"