/etc/openstack-dashboard/local_settings:

LOGGING = {
        'version': 1,
        # When set to True this will disable all logging except
        # for loggers specified in this configuration dictionary. Note that
        # if nothing is specified here and disable_existing_loggers is True,
        # django.db.backends will still log unless it is disabled explicitly.
        'disable_existing_loggers': False,
        'handlers': {
            'null': {
                'level': 'DEBUG',
                'class': 'django.utils.log.NullHandler',
                },
            'console': {
                # Set the level to "DEBUG" for verbose output logging.
                'level': 'DEBUG',
                'class': 'logging.FileHandler', # <-------------------------- I changed this
                'filename': '/tmp/horizon.log', # <-------------------------- And this
                },
            },
        'loggers': {
            # Logging from django.db.backends is VERY verbose, send to null
            # by default.
            'django.db.backends': {
                'handlers': ['console'],
                'propagate': False,
                },
            'horizon': {
                'handlers': ['console'],
                'propagate': False,
            },
            'novaclient': {
                'handlers': ['console'],
                 'propagate': False,
            },
            'keystoneclient': {
                'handlers': ['console'],
                'propagate': False,
            },
            'nose.plugins.manager': {
                'handlers': ['console'],
                'propagate': False,
            }
        }
}