Make Generator script more user friendly

For now; generator script embedded the execution of the container, but there is several limits:

• nuxeocli: naming could be better
• first parameter is the generator name can also be improved to have a easier input; like:
  nuxeocli nuxeo:samples xxx could be nuxeocli samples xxx to prevent from writing a lot of `nuxeo`
• block user that are trying to use the generator as sudoers (with an env variable to by pass the check)
• display help about generators (default, samples, hotreload, etc.)
• check if the docker image needs to be updated