Support Opentracing with Datadog - part 1

[ellieayla]

What this PR does / why we need it: Adds Datadog open tracing library to the Nginx container. This is a prerequisite of PR #3766, pointed out by @aledbf

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes half of #3752

Special notes for your reviewer: See PR #3766 for the configuration-generation half of this change

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[ellieayla]

e2e tests at https://travis-ci.org/kubernetes/ingress-nginx/jobs/493567349 failed, though I don't see how those failures might relate to this change. If a problem was introduced I would have expected nearly all tests to fail. Though perhaps only those which depended on the container building ok failed. Or those are the phase 2 tests.

@aledbf can you provide some guidance? Did I misunderstand the way I needed to split the PR?

Edit: That test passed. I must have been looking at some other test job 14h ago.

[ellieayla]

/check-cla

[aledbf]

@alanjcastonguay please squash the commits

[ellieayla]

Force-pushed a squashed version of previous commits to alanjcastonguay:feature/datadog-opentracing-part1

[ellieayla]

Cool; tests passed on the squashed variant. @aledbf does part 1 look good?

[aledbf]

@alanjcastonguay yes, I am building the image to test PR#2

[aledbf]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

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

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