Updated Openstack to terraform 0.12 (#5062)

* update openstack to terraform 0.12(.5)

* replace cluter.tf with cluster.tfvars

* update README.md to terraform 0.12

* update Openstack CI tests to use terraform 0.12

* specify terraform version in openstack README

* gitlab CI to copy cluster.tfvars in case of openstack provider

* The terraform/openstack dynamic inventory can read
tfstate v4 (generated by terraform 0.12) and convert them internally
ro v3 (as generated by terraform 0.11.x).

Additionally the script has been updated to Python 3.