Allow ECR Credential Helper to be disabled

[petetomasik]

Fixes #1599

• Adds new EnableECRCredentialHelper parameter that allows the Amazon ECR Credential Helper to be disabled in the ECR plugin. Defaults to true.
• Allows for pipeline-level overrides using the BUILDKITE_PLUGIN_ECR_CREDENTIAL_HELPER env var:

steps:
  - label: ":docker: Docker Build"
    env:
      BUILDKITE_PLUGIN_ECR_CREDENTIAL_HELPER: "false"  # Override Stack param
      command: "docker build ."

[scadu]

@petetomasik tests are failing with /etc/buildkite-agent/hooks/environment: line 112: ECR_CREDENTIAL_HELPER_ENABLED: unbound variable.
I reckon those should be set in packer/linux/conf/bin/bk-install-elastic-stack.sh, at least that seem to be the regular place.