Automatic merge from submit-queue (batch tested with PRs 51902, 52718, 52687, 52137, 52697). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>..

Refactoring and improvements for iSCSI and FC storage plugins

**What this PR does / why we need it**:

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.

**Which issue this PR fixes** : No related issues.

**Special notes for your reviewer**:

/cc @rootfs @jsafrane 

**Release note**:

```
NONE
```