Set logconfig default to paster more trivially

Rather than using fileConfig twice and risking different defaults
and behavior around disable_existing_loggers (ref #902), simply
set the default logging config file to be the paster config file
if it has a logger section and let glogging set up the rest.
Merge pull request #941 from benoitc/unify-paste-logging

Set logconfig default to paster more trivially