This PR makes following changes.

- Simplify volume tearDown path for iSCSI and FC using
  util.UnmountPath().
- Log lastErr during iscsi connection
  If iscsid fails to connect second portal, currently
  the error is ignored silently. The lastErr should be
  logged to find the root cause of problem.
- Remove iscsi plugin directory after iscsi connection
  is successfully closed.