Make dataflow on_delete a virtual field

[c2thorn]

Fixes hashicorp/terraform-provider-google#6301
updating on_delete would either cause the batch job to be destroyed or the stream to be updated-by-replacement. To prevent this, we need to check if on_delete is our only change by update time.

Release Note Template for Downstream PRs (will be copied)

`dataflow`: fixed an issue where `google_dataflow_job` would try to update `max_workers`

`dataflow`: fixed an issue where updating `on_delete` in `google_dataflow_job` would cause the job to be replaced

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 124 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 124 insertions(+), 1 deletion(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 126 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 126 insertions(+), 1 deletion(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 127 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 127 insertions(+), 1 deletion(-))

[danawillow]

Is this true for sure? We have other resources with maps that we just do a HasChange on the whole thing for.

[c2thorn]

After some digging, turns out that this is just needed for the labels field due to the resourceDataflowJobLabelDiffSuppress. When we decide to suppress a key, the parent field still reports that it has a change. Terraform plan will show no changes, but d.HasChange("labels") = true. I ran into this disparity between the labels parent/keys and incorrectly assumed that it could happen to other maps.

I can't find ways to clear the change during the diff suppress, so I believe the iteration is still needed for labels. I modified the condition and comments to be specific for the labels field.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 128 insertions(+), 4 deletions(-))
Terraform Beta: Diff ( 2 files changed, 128 insertions(+), 4 deletions(-))