add os type to clusterdefinition.md

[CecileRobertMichon]

What this PR does / why we need it:

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

Special notes for your reviewer:

If applicable:

• documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

[codecov]

Codecov Report

Merging #2806 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2806   +/-   ##
=======================================
  Coverage   46.93%   46.93%           
=======================================
  Files          86       86           
  Lines       12786    12786           
=======================================
  Hits         6001     6001           
  Misses       6231     6231           
  Partials      554      554

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[jackfrancis]

lgtm