chore: switch from drop-ins to drop_ins, fix: templating of DropIns

[adberger]

What this PR does / why we need it:

Changes originally from #475

• CoreOS Cloud Init is not reading the drop-ins sections in cloud-config.yml since Flatcar 3975.2.0, so we switch to drop_ins.
• CoreOS Cloud Init is not correctly templating the drop-ins sections because of a wrong condition ({{ if $unit.Content }} instead of {{ if $unit.DropIns }} or {{ with $unit.DropIns }})

Which issue(s) this PR fixes:

Fixes #1567

What type of PR is this?

/kind bug

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

NONE

Documentation:

NONE

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[kron4eg]

/ok-to-test

[kron4eg]

osm-fail.txt
here's the fail log

[adberger]

@kron4eg

I found and fixed the error.
I also found a second conditional error.

I refrained from adding both drop-ins & drop_ins, since @jepio said:

Yes, the canonical name of that section was always drop_ins but the old yaml parser performed automatic conversion for drop-ins. This isn't supported in newer parser versions. It's also unfortunate that the examples in docs used drop-ins.

So it should be backwards compatible.

[kron4eg]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 1a3b441930423518044da598bbc63d1f4c0118f2

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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