How to provide log_config option in keystone.conf

If I provide log_format option in keystone.conf I get this error:

Traceback (most recent call last):
File "/var/keystone-http/main", line 25, in <module>
  options = deploy.appconfig('config:%s' % CONF.config_file[0])
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 261, in appconfig
  global_conf=global_conf)
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 296, in loadcontext
  global_conf=global_conf)
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 320, in _loadconfig
  return loader.get_context(object_type, name, global_conf)
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 413, in get_context
  defaults = self.parser.defaults()
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 68, in defaults
  defaults[key] = self.get('DEFAULT', key) or val
File "/usr/lib/python2.7/ConfigParser.py", line 623, in get
  return self._interpolate(section, option, value, d)
File "/usr/lib/python2.7/dist-packages/paste/deploy/loadwsgi.py", line 75, in _interpolate
  self, section, option, rawval, vars)
File "/usr/lib/python2.7/ConfigParser.py", line 669, in _interpolate
  option, section, rawval, e.args[0])
terpolationMissingOptionError: Error in file /etc/keystone/keystone.conf: Bad value substitution:
section: [DEFAULT]
option : log_format
key : asctime
rawval : %(asctime)s %(levelname)8s [%(name)s] %(message)s

Is there any way to escape %()s for paste pastedeploy?