Fix config interpolation for some builders

OpenStack uses official OpenStack client

documentation: subnet_id is required for non-default VPC

* b-amazon-error-on-ssh-error:
  builder/amazon: Properly return error code on ssh errors

provisioner/shell: set -e for inline [GH-2069]

builder/digitalocean: user data support [GH-2113]

provisioner/shell: uploaded script should be 0755 [GH-1708]

ignore non-exe plugins on Windows [GH-2173]