fix: ensure /mnt is ext4

[jackfrancis]

Reason for Change:

This PR follows on from #3273 to ensure that /mnt is mounted as ext4 filesystem type via cloud-init.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[anhvoms]

You might want to add a comment that this is due to cloud-init bug:
https://bugs.launchpad.net/cloud-init/+bug/1879552

The resource disk should get partitioned, formatted, mounted correctly regardless of the presence of datadisks configuration. However, it looks like it only got partitioned and mounted, not formatted correctly

[codecov]

Codecov Report

Merging #3297 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3297   +/-   ##
=======================================
  Coverage   71.87%   71.87%           
=======================================
  Files         147      147           
  Lines       25529    25529           
=======================================
  Hits        18348    18348           
  Misses       6043     6043           
  Partials     1138     1138           

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	42.19% <ø> (ø)

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[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

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