Add external storage support for etcd

[gottwald]

What this PR does / why we need it:

This adds DataDisks support for DOMachines which can be used to host etcd data on block storage volumes.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #197

Special notes for your reviewer:
This does not include resizing support. Should be followed up by another PR to introduce it.

Release note:

Add DataDisks support for DOMachines which can be used to host etcd data on block storage volumes.

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[k8s-ci-robot]

Hi @gottwald. Thanks for your PR.

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[timoreimann]

/ok-to-test

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[MorrisLaw]

This is looking good! Left a few comments.

[MorrisLaw]

/ok-to-test

[prksu]

/assign

I'll take a look into this one

[prksu]

Hi @gottwald, thanks for tackling this one. My apologize for long delay.
I left some comment

[gottwald]

@prksu PTAL

[prksu]

already tested and all functionality is good. let's add the e2e test for this feature as the follow up.

thanks, @gottwald !!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gottwald, prksu

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