Fix state_path in tests

Fix state_path in tests

cfg.CONF.state_path is set to a random temporary directory
in neutron.tests.base:BaseTestCase.setUp. This value was then
over written in neutron.tests.unit.__init__. Tests that need
to read or pass cfg.CONF.state_path were getting the directory
from which the tests were running and not the temporary directory
specially created for the current test run. Note that the usage
of state_path to set lock_path, dhcp state path and the likes
was working as expected, and was not affected by this bug.

Closes-Bug: #1363058
Change-Id: Ib45f663fadaf0f3b4a79a0db4128822187b61ecc