Add support for Terraform 0.12

[ivankorn]

Please, note: 0.12 support for managed_instance_group example depends on this. Module to be updated once this task is done. Linked:

• module container-vm
• terraform-google-lb-http

[morgante]

This module will be deprecated, so we will not migrate it to v0.12.

Please replace usage with this module: https://github.com/terraform-google-modules/terraform-google-vm/tree/master/modules/mig