text: Title: Improve template logging and cookbook
  Body: While using alembic I came across two areas that I think could be improved:
  
  - in the template `evn.py` before setting the log config we should check if it's not already setup.
    I think that we can do it by checking if the root logger has an handler configured.
    This is useful if alembic is used programmatically, since it avoids resetting the program logging config.
    I guess that an option may comment may be enough for this, if the update in the template is too much
  - Add a cookbook recipe on how to add a non nullable column without a server default value