Use a temp dir for CONF.state_path

external_pids will defaults to $state_path(/var/lib/neutron)/external/pids
which isn't accessible when running unit tests so use a tempdir for
CONF.state_path. This bug arose when oslo.config 1.3.0 came out which
fixed bug #1282250 (I4324bda6f3de3cdf05f8eb3cf16052c04018dec8) and
caused this issue.

Change-Id: I253c5ea85c51eff94399da0b9462d10a9c509a0f
Closes-Bug: #1301027