fix(controller): Fix template resolution for step groups. Fixes #1868

[dtaniwaki]

Fixes #1868 #1876

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234". Why? for the release notes.
• Optional. I've added My organization is added to the README.
• I've signed the CLA and required builds are green.

I have fixed the wrong use of template context for step groups and also set a template scope in every node just in case it might be useful in the future.

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[dtaniwaki]

I also fixed an error of parallelism check caused by template scope which can be reproduced with the following manifests.

apiVersion: argoproj.io/v1alpha1
kind: WorkflowTemplate
metadata:
  name: nested-steps-with-params
spec:
  templates:
    - name: main
      steps:
        - - name: main-sub
            template: sub
    - name: sub
      steps:
        - - name: sub-print-string
            template: print-string
            arguments:
              parameters:
               - name: letter
                 value: '{{item}}'
            withParam: '["x", "y", "z"]'
    - name: print-string
      inputs:
        parameters:
         - name: letter
      container:
        image: alpine:3.6
        command: [sh, -c]
        args: ["echo {{inputs.parameters.letter}}"]
---
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: nested-steps-with-params-
spec:
  entrypoint: main
  templates:
    - name: main
      templateRef:
        name: nested-steps-with-params
        template: main

[alexec]

Do we need new tests for this?

[dtaniwaki]

@alexec Added.

[ddseapy]

@alexec can this get merged before the release candidate? If so we should be able to help test the rc.

[alexec]

@dtaniwaki I'll approve this, I think you need to do git merge master?

[alexec]

I think this might fix #2056