Fix google_compute_instance_template crash

[c2thorn]

Fixes hashicorp/terraform-provider-google#5018

ForceNew logic for source_image was handled with in a custom diff. When source_image was a self_link to another resource, the custom diff would ForceNew the field before the dependent resource was resolved. This would lead to the SDK trying to sort out a change to a nil value.

To fix this, I took the forcenew logic out of the custom diff, and just set the field to ForceNew in the schema. This way, Terraform will resolve the ForceNew normally. The remaining custom diff acts as a more complicated diffSuppress. I'm not sure if it can be rewritten as an actual diffSuppress, since it takes in the meta object, where diffSuppress does not.

Also added a test that tests this specific scenario since it wasn't covered in the current set of tests.

Release Note Template for Downstream PRs (will be copied)

Fixed a scenario where `google_compute_instance_template` would cause a crash.

[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, 67 insertions(+), 9 deletions(-))
Terraform Beta: Diff ( 2 files changed, 67 insertions(+), 9 deletions(-))

[danawillow]

The original PR to add this (hashicorp/terraform-provider-google#1995) has the note:

CustomizeDiff was having trouble (I think?
More on this below) on setting a field as not ForceNew once the field
was already set, so I moved the decision for whether a field was
ForceNew or not into CustomizeDiff.

I assume that's all fine and good now?

[c2thorn]

The original PR to add this (terraform-providers/terraform-provider-google#1995) has the note:

CustomizeDiff was having trouble (I think?
More on this below) on setting a field as not ForceNew once the field
was already set, so I moved the decision for whether a field was
ForceNew or not into CustomizeDiff.

I assume that's all fine and good now?

Super interesting, but I don't believe it's a problem now. The diff.clear(key) properly un-sets the ForceNew, and a diff doesn't show.