[fix] respect vlan interface if defined in linodemachine

[AshleyDumaine]

What this PR does / why we need it: Currently we can't define the VLAN interface to be on something other than eth0. This respects what VLAN interface is defined in the LinodeMachine if any.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.85%. Comparing base (53f8af3) to head (565c74a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
controller/linodemachine_controller_helpers.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
- Coverage   64.99%   64.85%   -0.14%     
==========================================
  Files          79       79              
  Lines        5656     5663       +7     
==========================================
- Hits         3676     3673       -3     
- Misses       1708     1717       +9     
- Partials      272      273       +1     

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[rahulait]

lgtm