Allow controller to watch a specified namespace

[fraenkel]

Changes

Add a command line option to optionally watch a specific namespace. The
default is to watch all namespaces.

Fixes #2111

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[tekton-robot]

Hi @fraenkel. Thanks for your PR.

I'm waiting for a tektoncd 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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[vdemeester]

/meow

/cc @abayer

[tekton-robot]

@vdemeester:

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[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[abayer]

/lgtm

This feels exactly right to me. =)