Add support for default pod template

[eddycharly]

This PR adds support for a default pod template through the default ConfigMap.

This is useful when one wants to apply a pod template to all task and pipeline runs, for example specifying a node selector, a security policy, etc...

When a pod template is specified for a PipelineRun or TaskRun it takes precedence over the default pod template.
Therefore, the default pod template only applies to PipelineRun or TaskRun that don't have a pod template.

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• Includes tests (if functionality changed/added)
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[tekton-robot]

Hi @eddycharly. 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]

/ok-to-test

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/config/default.go	84.6%	90.9%	6.3

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vdemeester

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[vdemeester]

/test tekton-pipeline-unit-tests

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/config/default.go	84.6%	90.9%	6.3
test/builder/pipeline.go	88.2%	87.3%	-0.9
test/builder/task.go	82.3%	81.1%	-1.1

[tekton-robot]

The following is the coverage report on pkg/.
Say /test pull-tekton-pipeline-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/apis/config/default.go	84.6%	90.9%	6.3
test/builder/pipeline.go	88.2%	87.3%	-0.9
test/builder/task.go	82.3%	81.1%	-1.1

[vdemeester]

/lgtm