Support GCS backend prefixes without trailing /

[negz]

I initially thought listing workspaces with the GCS backend was broken, but it turns out I just omitted the trailing / in my prefix path. Creating new workspaces worked fine, but I was unable to list or select existing workspaces:

$ grep -A4 gcs main.tf                        
  backend "gcs" {                                                        
    project = "planet-k8s-staging"                                      
    bucket  = "terraflop-state"                            
    prefix  = "terraflop/etcd"          
  }                                                                                 
$ tf version                            
Terraform v0.11.0-beta1 (23caaf9feb50d566329f5b4aa07a57da8cd6f448)     
+ provider.google (unversioned)                                
+ provider.ignition v1.0.0                                                                  
+ provider.local v1.0.0                                         
+ provider.random v1.0.0                                                 
+ provider.template v1.0.0                                         
+ provider.tls v1.0.0                                                  
                                                                           
$ tf workspace list                                         
  default                                                                
$ gsutil ls gs://terraflop-state/terraflop/etcd         
gs://terraflop-state/terraflop/etcd/default.tfstate                        
gs://terraflop-state/terraflop/etcd/etcd.tfstate                            
gs://terraflop-state/terraflop/etcd/herp.tfstate    
gs://terraflop-state/terraflop/etcd/koobz.tfstate  
gs://terraflop-state/terraflop/etcd/negz.tfstate
gs://terraflop-state/terraflop/etcd/shrd.tfstate

I considered using a ValidateFunc to enforce the trailing /, but it seemed like a better experience for the user to fix it behind the scenes.

$ TF_ACC=1 GOOGLE_PROJECT=planet-k8s-staging go test .
ok      github.com/hashicorp/terraform/backend/remote-state/gcs 21.359s

[apparentlymart]

Hi @negz! Thanks for working on this.

This seems like a fine idea to me. Agreed that it's better to just "make this work" rather than forcing the user to add the slash manually, since it seems pretty clear what the user intent is.

[ghost]

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