importing the pipeline api pollutes flags

[fejta]

Importing the pipeline api causes a bunch of global flags to get introduced, for example:

pipeline/pkg/apis/pipeline/v1alpha1/git_resource.go

Lines 33 to 35 in 5e3276d

	gitImage = flag.String("git-image", "override-with-git:latest",
	"The container image containing our Git binary.")
	)

Ideally people should be able to consume this api definition without polluting their flag namespace.
Can we move these flags into the pipeline controller and/or move them inside an optionally called function rather than global vars / init statements which everyone has to run?

ref kubernetes/test-infra#14320

[vdemeester]

/assign
/kind feature