NetApp: Clean up lock file left behind by unit tests

The synchronization decorator added to the NetAppISCSIDriver class
creates a lock file named cinder/openstack/cinder-netapp_dfm
during the unit tests. This commit cleans it up.

Fixes bug 1135619

Change-Id: Iec41a0f23d4a1c6668bdcb451c2c02efd3f97500