Add variable to determine sidecar container position in container list

[Kashuab]

What does this PR do?

Adds a variable that allows users to determine if the container is prepended or appended. Appending the sidecar container instead of prepending helps for migration cases.

Motivation

Upon trying to deploy with this module, terraform apply was failing with the following error:

[
  {
    "@type": "type.googleapis.com/google.rpc.BadRequest",
    "fieldViolations": [
      {
        "description": "Revision template should contain exactly one container with an exposed port.",
        "field": "template.containers"
      }
    ]
  }
]

My hypothesis is that since the default behavior is to prepend the sidecar container into the service, Google was trying to update the existing main service container to be the sidecar (retaining the ports for some reason) while simultaneously creating a new app container to serve traffic.

Using an append strategy solved the error in our case, because it instead only creates a new container without ports by default.

Describe how you validated your changes

Ran terraform apply using the latest commit on the fork, and datadog_sidecar_position = "append" on both modules definitions and the issue was resolved.

Additional Notes

• This was branched off my other fix, see Use ternary instead of short-circuit condition, see #45 #46
• I have a feeling that the prepend default has a purpose. I'm curious why this is?
• TODO: Update documentation to include new variable, and explain why it's helpful