CI on G5K

This PR:

• add stage (manually trigger) in Gitlab CI to run tutorial or another Enoslib exp on G5K,
• need 4 new Gitlab CI variables:
  • LOGIN, type: variable, the login of the G5K account
  • PYTHON_G5K, type: file, the content of .python-grid5000.yaml
  • PUBLIC_KEY, type: file, content of the public key associated to the G5K account
  • PRIVATE_KEY, type: file, content of the private key associated to the G5K account

Thanks to @bjonglez for helping to debug.