fix: allow the specific image platform os/arch/variant to meet the requirement for a broader image platform os/arch

[chengfang]

Fixes #711

• change WantsPlatform() method to allow match of broader platform
• add unit tests for the above change
• improve logs to include platform variant so it's clear why certain match is false
• this fix causes some change of behavior in comparing options platforms vs platforms of available images, as shown in the changes to pkg/image/options_test.go

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.48%. Comparing base (40f260a) to head (2e4b9c0).

Files with missing lines	Patch %	Lines
pkg/registry/client.go	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
+ Coverage   73.43%   73.48%   +0.05%     
==========================================
  Files          31       31              
  Lines        3136     3142       +6     
==========================================
+ Hits         2303     2309       +6     
  Misses        695      695              
  Partials      138      138              

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[jannfis]

LGTM, great stuff!