Use openstack.common.lockutils module for locks in tox functional tests.

Tox test has been switched to use openstack.common.lockutils module
for interprocess locks. However neutron tox functional tests was
left behind probably because locking is not required by the existing
functional test cases. This change makes neutron tox functional tests
adopt openstack.common.lockutils for locking.

Closes-Bug: 1332380
Change-Id: I4d6076e5f7586a11ea5381f18774d4bab57b0081
Merge "Use openstack.common.lockutils module for locks in tox functional tests."