doc: Documenting how to configure s3 backend with env vars

[jeansebastienh]

Fix the following error if only SCW_ACCESS_KEY and SCW_SECRET_KEY are configured

$ terraform init                         
                                                                                                                       
Initializing the backend...                                                                                            
Backend configuration changed!                                                                                         
                                                                                                                       
Terraform has detected that the configuration specified for the backend  
has changed. Terraform will now check for existing state in the backends.      
                                                                                                                       
                                                                                                                       
                                                           
Error: error configuring S3 Backend: no valid credential sources for S3 Backend found.                                 
                                                                                                                       
Please see https://www.terraform.io/docs/backends/types/s3.html                                     
for more information about providing credentials.                                                                      
                                                                                                                       
Error: NoCredentialProviders: no valid providers in chain. Deprecated.                                                 
        For verbose messaging see aws.Config.CredentialsChainVerboseErrors