v0.13.x: fixes for rendering plugin systemd units

[davidmccormick]

Changed Machine.SystemdUnits to inline rather than needing contents->content.

Fixed issue rendering systemd unit templates.
Fixed issue when RemoteFileSpec contains Template and not content.
Refactored extras.go to clean up repetitive code.

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[codecov-io]

Codecov Report

Merging #1674 into v0.13.x will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff            @@
##           v0.13.x    #1674   +/-   ##
========================================
  Coverage    25.41%   25.41%           
========================================
  Files           98       98           
  Lines         5064     5064           
========================================
  Hits          1287     1287           
  Misses        3634     3634           
  Partials       143      143

Impacted Files	Coverage Δ
pkg/api/plugin.go	0% <ø> (ø)	⬆️
pkg/model/stack_new.go	33.86% <0%> (ø)	⬆️

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[tyrannasaurusbanks]

/lgtm & fixes the issues I was facing on my plugin migration

[omar-nahhas]

Thank you very much @davidmccormick, I have also tested it and the rendering works fine, would it be possible to extend the current tests on test/integration package to test the rendering of systemd units?