extra: add Terraform config files

For testing Tarantool with Jepsen we use virtual machines as they provides
better resource isolation in comparison to containers. Jepsen tests may need a
single instance or a set of instances for testing cluster.  To setup virtual
machines we use Terraform [1]. Patch adds a set of configuration files for
Terraform that can create required number of virtual machines in MCS and output
IP addresses to stdout.

Terraform needs some parameters before run. They are:

- id, identificator of a test stand that should be specific for this run, id
also is a part of virtual machine name
- keypair_name, name of keypair used in a cloud, public SSH key of that key pair
will be placed to virtual machine
- instance_count, number of virtual machines in a test stand
- ssh_key, SSH private key, used to access to a virtual machine
- user_name
- password
- tenant_id
- user_domain_id

These parameters can be passed via enviroment variables with TF_VAR_ prefix
(like TF_VAR_id) or via command-line parameters.

To demonstrate full lifecycle of a test stand with Terraform one needs to
perform these commands:

terraform init extra/tf
terraform apply extra/tf
terraform output instance_names
terraform output instance_ips
terraform destroy extra/tf

1. https://www.terraform.io/

Part of #5277