✨ Add feature gate environment variable for machine pool machines

[AndiDog]

What type of PR is this?

/kind documentation
/kind support

What this PR does / why we need it:

Allow enabling the feature #5319 with an environment variable and enable it by default in the suggested Tilt testing setup.

Checklist:

• squashed commits
• includes documentation
• includes emoji in title
• adds unit tests
• adds or updates e2e tests

Release note:

Add feature gate environment variable for machine pool machines

[k8s-ci-robot]

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[fiunchinho]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: d7d6de830c607fe1bdfe00ec0966c15cb5a0f7f2

[AndiDog]

/assign @dlipovetsky @nrb @Ankitasw for approval

[k8s-ci-robot]

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In response to this:

/assign @dlipovetsky @nrb @Ankitasw for approval

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