Automatic merge from submit-queue (batch tested with PRs 55112, 56029, 55740, 56095, 55845). 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>.

Updating vsphere cloud provider to support k8s cluster spread across multiple vCenters

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

vSphere cloud provider in Kubernetes 1.8 was designed to work only if all the nodes of the cluster are in one single datacenter folder. This is a hard restriction that makes the cluster not span across different folders/datacenter/vCenters. Users have use-cases to span the cluster across datacenters/vCenters.

**Which issue(s) this PR fixes** 
Fixes # https://github.com/vmware/kubernetes/issues/255

**Special notes for your reviewer**:
This is a change purely in vsphere cloud provider and no changes in kubernetes core are needed.

**Release note**:
```release-note
With this change
 - User should be able to create k8s cluster which spans across multiple ESXi clusters, datacenters or even vCenters.
 - vSphere cloud provider (VCP) uses OS hostname and not vSphere Inventory VM Name.
   That means, now  VCP can handle cases where user changes VM inventory name.
- VCP can handle cases where VM migrates to other ESXi cluster or datacenter or vCenter.

The only requirement is the shared storage. VCP needs shared storage on all Node VMs.
```

Internally tested and reviewed the code.

@tthole, @shaominchen, @abrarshivani