Support Opentracing with Datadog - part 2

[ellieayla]

What this PR does / why we need it:

Support Opentracing with Datadog, alongside Zipkin and Jaeger.

• Build libdd_opentracing.so from source, install it in the nginx container.
• Plumb configuration options from the ConfigMap down to the library via /etc/nginx/opentracing.json template.
• Add markdown documentation.

Which issue this PR fixes: fixes #3752

Special notes for your reviewer: I didn't sink a lot of time into this. I am open to alternative approaches/implementations, or improvement suggestions.

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[aledbf]

@alanjcastonguay please split this PR in two. In the first one change the content of images/nginx/* and increase the version to 0.77 and in the second one change the nginx image to use https://github.com/kubernetes/ingress-nginx/blob/master/Makefile#L64

This is because we need to build and publish the nginx image before being able to run the e2e tests

[ellieayla]

Thanks for the feedback, @aledbf. All the changes under images/nginx/* were in a single commit 248d9b3, so I cut a new branch from it and created PR #3767, then reverted that commit from here.

Added a pair of commits, one to each PR, for a merge conflict - I branched from before 0.76.

I am working on the CLA.

[ellieayla]

/check-cla

[aledbf]

@alanjcastonguay please squash the commits

[aledbf]

/lgtm
/approve

[aledbf]

@alanjcastonguay thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alanjcastonguay, aledbf

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