CI: Generate ssh key pair on the fly

There is litte reason to share an ssh key common to all CI jobs, so
generate one for each on the fly.

Also use plain-text cloud-init config instead of base64 for readability

tests/scripts/testcases_run.sh

@@ -23,7 +23,6 @@ if [ "${UPGRADE_TEST}" != "false" ]; then
   git checkout "${CI_COMMIT_SHA}" -- tests/
 fi
 
-export ANSIBLE_REMOTE_USER=$SSH_USER
 export ANSIBLE_BECOME=true
 export ANSIBLE_BECOME_USER=root