pkg/cli/admin/mustgather: Add --pull-always CLI option to override ImagePullPolicy.

[ljforet]

Fixes #2029

[openshift-ci]

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[openshift-ci]

Hi @ljforet. Thanks for your PR.

I'm waiting for a openshift member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[openshift-bot]

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/lifecycle stale

[openshift-bot]

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Mark the issue as fresh by commenting /remove-lifecycle rotten.
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/lifecycle rotten
/remove-lifecycle stale

[openshift-merge-robot]

PR needs rebase.

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