[bugfix] Fix value passing bug in New Experiment form

[orfeas-k]

While looking at the integration of our previous YAML editor (ace-editor), we found out that we do not take the appropriate actions in order to pass the customYaml input as part of the form it is in.

Steps to reproduce the issue:

• Write any kind of input in Metrics Collector > Kind: Custom > Editor
• Click Create or Edit and Submit YAML
  An error will come up since we haven't set up properly a new experiment but if in our browser we use Inspect > Network (tab) > create_experiment > postData > spec. We can see metricsCollectorSpec > source hasn't any updated data in the section below:
  (https://github.com/kubeflow/katib/blob/master/examples/v1beta1/metrics-collector/custom-metrics-collector.yaml#L18-L35)

    collector:
      kind: Custom
      customCollector:
        args:
          - -m
          - accuracy
          - -s
          - katib-db-manager.kubeflow:6789
          - -path
          - /katib/mnist.log
        image: kubeflowkatib/custom-metrics-collector:latest
        imagePullPolicy: Always
        name: custom-metrics-logger-and-collector
        env:
          - name: TrialNamePrefix
            valueFrom:
              fieldRef:
                fieldPath: metadata.name

This PR resolves the above bug by adding missing logic in New Experiment form in order to pass the value of the editor content in Metrics Collector tab, when Kind is set to Custom.

[kimwnasptd]

/lgtm
/approve

[google-oss-prow]

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