add plugins support for k8s imagepullpolicy

[novahow]

Tracking issue

https://github.com/flyteorg/flyte/issues/

Why are the changes needed?

Users should be able to configure the ImagePullPolicy they desire for created pods in k8s plugin.

What changes were proposed in this pull request?

Added a field ImagePullPolicy in flytek8s plugin.

How was this patch tested?

make test_unit

setting different values for image-pull-policy, including not specifying the value (i.e. no image-pull-policy key)

plugins:
  logs:
    kubernetes-enabled: true
    kubernetes-template-uri: http://localhost:30080/kubernetes-dashboard/#/log/{{.namespace }}/{{ .podName }}/pod?namespace={{ .namespace }}
    cloudwatch-enabled: false
    stackdriver-enabled: false
  k8s:
    image-pull-policy: Always
    default-env-vars:
      - FLYTE_AWS_ENDPOINT: http://flyte-sandbox-minio.flyte:9000
      - FLYTE_AWS_ACCESS_KEY_ID: minio
      - FLYTE_AWS_SECRET_ACCESS_KEY: miniostorage
  k8s-array:
    logs:
      config:
        kubernetes-enabled: true
        kubernetes-template-uri: http://localhost:30080/kubernetes-dashboard/#/log/{{.namespace }}/{{ .podName }}/pod?namespace={{ .namespace }}
        cloudwatch-enabled: false
        stackdriver-enabled: false

Not specified

IfNotPresent

Always

Never

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.11%. Comparing base (524b473) to head (ee3f1c2).
Report is 21 commits behind head on master.

❗ Current head ee3f1c2 differs from pull request most recent head c014394. Consider uploading reports for the commit c014394 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5167      +/-   ##
==========================================
+ Coverage   59.10%   59.11%   +0.01%     
==========================================
  Files         645      645              
  Lines       55574    55575       +1     
==========================================
+ Hits        32846    32853       +7     
+ Misses      20135    20129       -6     
  Partials     2593     2593              

Flag	Coverage Δ
unittests	59.11% <100.00%> (+0.01%)	⬆️

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[pingsutw]

Thank you! LGTM