Increase python unittest timeout to 40 minutes.

The python unittests (particularly python26) have been getting close to
the test timeout and occasionally go over. Increase the test timeout by
10 minutes for a timeout of 40 minutes. This is approximately the length
of time that tempest-devstack-gate-vm tests take to run so should not
increase the runtime for a change in the qate.

Change-Id: Ic2c015ad87968a6885c9bbc82fed5f37b3aee209
Reviewed-on: https://review.openstack.org/22397
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins