fix: Ignore all stackdriver errors if --stackdriver-only-when-available is set

[maxsmythe]

This is to handle issues where the heuristics in google.FindDefaultCredentials() falsely indicate stackdriver is present, leading to the Gatekeeper pod crash looping.

Signed-off-by: Max Smythe smythe@google.com

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
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[codecov-commenter]

Codecov Report

Base: 53.32% // Head: 53.44% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (3b8e36e) compared to base (512d97e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2304      +/-   ##
==========================================
+ Coverage   53.32%   53.44%   +0.11%     
==========================================
  Files         116      116              
  Lines       10174    10174              
==========================================
+ Hits         5425     5437      +12     
+ Misses       4327     4318       -9     
+ Partials      422      419       -3     

Flag	Coverage Δ
unittests	53.44% <ø> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...onstrainttemplate/constrainttemplate_controller.go	57.65% <0.00%> (+2.87%)	⬆️

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