Tezt/Cloud: make explicit the use of an ssh public key

Context:

tezt-cloud generates an ssh private key named after the "$TEZT_CLOUD-tf" filename. This generation is done lazily in the docker_build job (in jobs.ml), because the public key corresponding to this private key needs to be included in the docker image to allow the following ssh connections to the docker images.

This MR

This MR move the generation of the SSH key from the docker_build function earlier in the deployment process, which will be necessary to provision and to deploy on machines via ssh. It makes explicit the provided ssh_public_key argument to the multiple docker_build usages, making this function better fitting its name.