remove log_file option

Partly fix #591. Gunicorn should only bother writing its own log and let
the application handle the way it want to log its own errors.

By removing the log_file option we are making sure that someone won't
attempt to use gunicorn to do more whn it's about logging. also we are
making sure that the logs options are not overriden.
remove the import as well.
don't propagate logs

fix #574
don't try to redirect stdout/stderr to the logfile.

Logging to a logfile should be explicit and gunicorn should not try to
be smarter here. Partially fix #591
make it explicit that we only send gunicorn logs to syslog.

follows suggestion in #548
allows possibility to set the driver used in syslog for unix sockets

This changes allows a use to set the driver used to send the data over a
unix socket. 'dgram' for a dgram driver or 'stream' for a stream driver.
'stream' is the default. The change is documented in the settings
documentation.

fix #671