Improve template logging and cookbook

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

Start with the template env.py and inspect how logging is configured before the migration setup. Then review the cookbook structure and add a recipe covering a non-nullable column without a server default; done means both the logging guidance and cookbook example address the two requested cases.