Ingress: clean nginx, traefik and ambassador tests

[mayankshah1607]

A cleaner, refactored version of https://github.com/linkerd/linkerd2-conformance/pull/12

This PR includes:

• tackle traefik, nginx and ambassador ingress controllers, as gloo and contour have a slightly different testing process (which will be tackled in a separate PR).
• use controller YAML URL instead of storing it locally
• port-forward on the ingress controller pod instead of relying on external IP
• make test structure a lot simpler

I did not directly make changes to https://github.com/linkerd/linkerd2-conformance/pull/12 so that gloo and contour tests are not lost because of the refactoring process.

Signed-off-by: Mayank Shah mayankshah1614@gmail.com

[mayankshah1607]

Thanks @ihcsim ! I have made the changes. :)

[ihcsim]

This one should be ready to be merged.