text: Title: Not able to set kernel name for notebook
  Body: Hi,
  I am trying to use my default JupyterLab kernel for executing notebooks via Notebooker but I don't think this configuration is applied. I've tried following commands
  
  `notebooker-cli --notebook-kernel-name=python3`
  `notebooker-cli --notebook-kernel-name python3`
  
  
  Opening the Notebooker output for the executing notebook I see following kernelspec:
  `  "kernelspec": {
     "display_name": "notebooker_kernel",
     "language": "python",
     "name": "notebooker_kernel"
    }`
  
  but I expect it to be 
  `  "kernelspec": {
     "display_name": "Python 3",
     "language": "python",
     "name": "python3"
    }`
  
  
  