Add vSphere external CCM

[maxilampert]

What this PR does / why we need it:
Adds vShpere external ccm controller.
Example kubeone config used for testing. Tested with current version of vCenter and ESXi 7.

apiVersion: kubeone.io/v1beta1
kind: KubeOneCluster
versions:
  kubernetes: '1.19.3'
cloudProvider:
  vsphere: {}
  external: true
  cloudConfig: |+
    global:
      port: 443
      # set insecureFlag to true if the vCenter uses a self-signed cert
      insecureFlag: true
      # settings for using k8s secret
      secretName: vsphere-ccm-credentials
      secretNamespace: kube-system
    # vcenter section
    vcenter:
      tenant-example:
        server: vcsa.example.com
        datacenters:
          - DC

Which issue(s) this PR fixes:
Implements #616 for vSphere controller
Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Add vSphere external CCM

[kubermatic-bot]

Hi @maxilampert. Thanks for your PR.

I'm waiting for a kubermatic member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[kron4eg]

Thanks @maxilampert!

/ok-to-test

[kron4eg]

/test pull-kubeone-lint
/test pull-kubeone-test

[kron4eg]

/retest

[kron4eg]

/retest

[kron4eg]

/lgtm

@xmudrii PTAL

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 8577d7b30ff01cc89b2c013ffd860073221a7efb

[xmudrii]

I've left some nits, but other than that, it looks very good.

[kron4eg]

Thank you!
/lgmt
/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 54a903d9f0941b58fca70698b68e82e9983e5dc0

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg, maxilampert

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