Improve golden planfile test maintainability

[kmoe]

The golden plan file used for Plan tests is specific to OS and Terraform version. Make this easier to maintain across development environments.

[kmoe]

Some options for doing this:

1. Make a script that will regenerate the plan files for a given Terraform version and OS, and have devs run this whenever they bump a Terraform version. Script could use Docker to run things under different OSs.
2. Make a custom CI step, with manual trigger, which can do the same and push the new planfiles to the relevant PR branch.
3. Relax the check on plan files, so the TerraformVersion doesn't have to match for tests such as TestShowPlanFile013 to pass. The test will still break if the plan file format changes in any other way, which is fine. Probably best in combination with (1) or (2), so the planfiles can be regenerated when the format does change.

[kmoe]

Step 3 is done, but this is not sufficient, because 0.13 is even stricter about planfile versions: in 0.13 terraform show itself will error (see #93):

--- FAIL: TestShowPlanFile013 (1.56s)
    --- FAIL: TestShowPlanFile013/non_default_planfile_013-0.13.5 (1.56s)
        util_test.go:104: [INFO] running Terraform command: /tmp/tfinstall829262588/v-0.13.5/terraform init -no-color -force-copy -input=false -lock-timeout=0s -backend=true -get=true -get-plugins=true -lock=true -upgrade=false -verify-plugins=true
        util_test.go:104: [INFO] running Terraform command: /tmp/tfinstall829262588/v-0.13.5/terraform show -json -no-color planfilefoo
        show_test.go:311: 
            Error: Invalid plan file
            
            Failed to read plan from plan file: plan file was created by Terraform 0.13.0,
            but this is 0.13.5; plan files cannot be transferred between different
            Terraform versions.