Fix No known host error message for DCR v2

[larohra]

Description

Currently, if any step fails in the DCR v2 run, the Install SSH PostJob shows this error - ##[error]Error: ENOENT: no such file or directory, unlink '/home/vsts/.ssh/known_hosts'

This error is unrelated to the actual failure and might cause confusion during debug.

This PR fixes this issue.

Issue #

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PR information

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• New Unit tests were added for the changes made

Quality of Code and Contribution Guidelines

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[larohra]

sample run - https://dev.azure.com/cplatruntime/WALinuxAgent/_build/results?buildId=1208&view=results

[codecov]

Codecov Report

Merging #2554 (57f0b30) into develop (713633a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2554   +/-   ##
========================================
  Coverage    71.76%   71.76%           
========================================
  Files          102      102           
  Lines        15314    15314           
  Branches      2426     2426           
========================================
  Hits         10990    10990           
  Misses        3834     3834           
  Partials       490      490           

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