Tezt/Cloud: add a temporary configuration step

Several engineers reported that they had to run an additional command not described in the current Tezt-cloud documentation in order to deploy virtual machines.

This request adds this command to the Tezt-cloud documentation so that other engineers don't get blocked on the potential deployment error:

terraform  '-chdir=tezt/lib_cloud/terraform/vm' init

Note: the Tezt-cloud scripts should already include this step transparently when deploying virtual machines (see #8024 (closed)).

Manually testing the MR

If it's your first time running Tezt-cloud, then follow the current documentation. When deploying a virtual machine (with dune exec tezt/tests/cloud/main.exe -- BASIC health -v for instance), you might end up with this error:

[error] terraform exited with code 1 (full command: terraform '-chdir=tezt/lib_cloud/terraform/vm' workspace select default)

Try again after running terraform '-chdir=tezt/lib_cloud/terraform/vm' init and you should be alright.