Explicitly import state_path opt in tests.base

This wasn't explicitly needed in the unit tests because
neutron.common.config was already loaded, but this broke for the
functional tests (which are non-voting) so add a CONF.import_opt line so
functional tests know where to find the definition for state_path.

Closes-Bug: #1322440

Change-Id: I5b87a75317e2ba670adaa59b251bfa860920fd61
(cherry picked from commit f6dfc3a901ce7536a72e6159583051e9bcaf8d8f)