Add support for 'staging' of draft service-versions.

[kpfleming]

This adds a 'stage' attribute to both fastly_service_vcl and
fastly_service_compute resources, with a default of 'false' (no change
to existing behavior).

If 'stage' is set to 'true', then 'plan' operations which make changes
to service versions will also 'stage' those service versions, whether
they are also activated or not. The expected usage is to have 'stage =
true' and 'activate = false', so that the provider will clone when
necessary to make draft versions, apply changes to those draft
versions and stage them, but otherwise leave them alone. When 'stage =
true' an existing draft version will not be cloned in order to apply
additional changes to it, they will be applied to that draft version.

Additional implementation notes:

• Added a new function in fastly_test.go to render templates (from the
  text/template module), which can be used by acceptance tests for
  generating HCL. The acceptance tests for the staging feature use
  templates.

• Moved a function from resource_fastly_service_vcl_test.go to
  fastly_test.go, refactored it, and fixed bugs in it. This function
  is used in acceptance tests to compare the list of expected backends
  for a service to the list of actual backends for the service.

• Added golang-set as a new dependency, which is used in the function
  mentioned in the last bullet.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

• What is the user impact of this change? - it adds new functionality

[philippschulte]

This took me a while to wrap my head around. The changes look good and make sense now.

I think it's worth to mention somewhere in the provider docs that the following doesn't make sense:

resource "fastly_service_vcl" "test" {
  ...
  
  activate = true
  stage = true
  
  ...
  }

Specially because activate = true is the default. Why would you activate staging and production at the same time. Since we don't check this during the plan I think it would be good to to have a notice in the docs which points out that when you use stage to set activate to false. Maybe the Guides section would be the right place or even under the stage field description itself.

[kpfleming]

A new 'Activation and Staging' section has been added to the docs for both Delivery and Compute services.

[anthony-gomez-fastly]

LGTM

[philippschulte]

Thank you for adding detailed docs!

[mattcone]

LGTM! I've left some comments regarding some minor formatting issues.

[kpfleming]

OK, this needs final check-offs from @philippschulte and @anthony-gomez-fastly and it's ready to be merged. Thanks to everyone for the feedback!