make a destroy plan work the same as destroy

When working on an existing plan, the context always used walkApply,
even if the plan was for a full destroy. Mark in the plan if it was
icreated for a destroy, and transfer that to the context when reading
the plan.

terraform/context.go

@@ -539,6 +539,7 @@ func (c *Context) Plan() (*Plan, error) {
 	var operation walkOperation
 	if c.destroy {
 		operation = walkPlanDestroy
+		p.Destroy = true
 	} else {
 		// Set our state to be something temporary. We do this so that
 		// the plan can update a fake state so that variables work, then

terraform/plan.go

@@ -71,6 +71,9 @@ type Plan struct {
 	// Backend is the backend that this plan should use and store data with.
 	Backend *BackendState
 
+	// Destroy indicates that this plan was created for a full destroy operation
+	Destroy bool
+
 	once sync.Once
 }
 
@@ -100,6 +103,7 @@ func (p *Plan) contextOpts(base *ContextOpts) (*ContextOpts, error) {
 	opts.Module = p.Module
 	opts.Targets = p.Targets
 	opts.ProviderSHA256s = p.ProviderSHA256s
+	opts.Destroy = p.Destroy
 
 	if opts.State == nil {
 		opts.State = p.State