Allow output parameters with .value, not only .valueFrom

[Ark-kun]

Fixes #1329

This small PR just tweaks the validator to enable a valid user scenario.

Scenario:
Upstream tasks used {{pod.name}} in to generate some data storage location (e.g. a BigTable table).
Downstream component wants to know the {{pod.name}} of upstream.

Currently doing this requires explicitly writing the pod name to file and then telling Argo to pick it up.

This can be made simple by just allowing output parameters with explicitly set value (instead of valueFrom). This only requires a small validator change so that it does not return error when .valueFrom is missing, but .value is specified.

Example:

Template that does not output pod name:

container:
    command: [myprog.py, arg1, arg2, arg3]

Template that outputs pod name (current workaround):

container:
    command: [sh, -c, 'echo "{{pod.name}}" > /tmp/pod_name.txt && "$*"',
        myprog.py, arg1, arg2, arg3
    ]
outputs:
    parameters:
    - name: pod-name
      valueFrom:
          path: /tmp/pod_name.txt

Template that outputs pod name (after this PR):

container:
    command: [myprog.py, arg1, arg2, arg3]
outputs:
    parameters:
    - name: pod-name
      value: "{{pod.name}}"

The workaround might not be even possible in cases where we do not know the command of the container and cannot inject anything there.

[Ark-kun]

@xianlubird does this look good?

[xianlubird]

Seems LGTM.
Pls @sarabala1979 take a look

[Ark-kun]

@jessesuen, can you please take a look when you have time? This PR is a small validator-only change.

[Ark-kun]

Gentle ping

[Ark-kun]

@sarabala1979 @jessesuen Can you please take a look?

[sarabala1979]

LGTM

[Downchuck]

@Ark-kun The work-around (before this patch) doesn't give the desired results for pod.name when running in a task group pod from withParam. My guess is that this patch actually gives the desired results. But heads up on that anyway.