Config lockutils to use a temp path for tests.

 * Tests for iptables_manager were leaving lockfiles in the source
   tree due to the default lock path being in the openstack common
   tree.  This patch sets the lock_path option to an empty string
   for unit tests to ensure that a temporary location is used
   instead.

Change-Id: I5d36dca8cfeba991dfe0803e536d3d74dd2ffe7f
Merge "Config lockutils to use a temp path for tests."