fix: option to disable base env var injection

[simonostendorf]

Fixes #734

#486 introduced the environment variables NAMESPACE, HOST_IP and POD_IP which are not needed in the code (as I checked it) and are only there as "nice to have".

In newly created clusters where the cloud controller manager is not running (because the agent injector should be used to inject secrets into the cloud controller manager) the nodes do not have a host ip address set. So the pods cannot start because of the error Error: host IP unknown;.

This PR adds the ability to disable the injection of these basic env vars with a new annotation called vault.hashicorp.com/agent-no-base-env-vars=true.

I hope I did all relevant code changes, I am new to this project.

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

[simonostendorf]

I found another "dirty" fix when using the following annotations:

  vault.hashicorp.com/agent-json-patch: |
    [
      { "op": "remove", "path": "/env/0" },
      { "op": "remove", "path": "/env/0" },
      { "op": "remove", "path": "/env/0" }
    ]
  vault.hashicorp.com/agent-init-json-patch: |
    [
      { "op": "remove", "path": "/env/0" },
      { "op": "remove", "path": "/env/0" },
      { "op": "remove", "path": "/env/0" }
    ]