📖 Add metal3 host operator.

[sailorvii]

What this PR does / why we need it:
In some scenarios, customers like to acquire an empty bare-metal host or a group of hosts.
Furthermore, users hope that host could pre-install the customized application.
It's a pure bare-metal management platform without guest Kubernetes.

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 ##649

[metal3-io-bot]

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[metal3-io-bot]

Hi @sailorvii. Thanks for your PR.

I'm waiting for a metal3-io 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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[kashifest]

This is not the right place to open up proposals. Usually in metal3.io we first discuss all the proposals in https://github.com/metal3-io/metal3-docs

[sailorvii]

https://github.com/metal3-io/metal3-docs

Thanks. I'll move it there.
The new place is metal3-io/metal3-docs#268.