Drop strategy field in favour of annotation

[enxebre]

There's only two potential use case strategies today: delete and reboot.
Reboot is not yet supported at the machine API level but is rather a baremetal specific process.
Delete is not a good fit on baremetal.
As there's no specific strong use case or clear path forward yet for strategies reusable across multiple environments we don't want to bake this in the API just yet. Instead we are adding an annotation for the strategy on the MHC that can be leveraged for plugin strategies.

• Introduce healthchecking.openshift.io/strategy annotation
• Drop spec.RemediationStrategy

Needs #414

[enxebre]

/retest

[enxebre]

/retest

[enxebre]

/retest

[enxebre]

/retest

[enxebre]

/retest

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[enxebre]

This is a follow up for #414
I'm fast merging this to enable farther development and put up the PR for:
Remove feature gate #416

/approve

[openshift-ci-robot]

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