As I mentioned in the comment, I've never seen a config file with the section headings on the same line as config option:
[DEFAULT] log_file = /var/log/keystone/keystone.log
I've also seen it like this instead:
[DEFAULT]
log_file = /var/log/keystone/keystone.log
However, I have no idea if this is related to your issue.
To enable debug-level logging, make sure you set the following in /etc/keystone/logging.conf:
[logger_root]
level=DEBUG
Then try running keystone manually instead of as a service:
$ sudo -u keystone bash
$ HOME=/var/lib/keystone keystone-all --debug
Hopefully you'll see a relevant error message on standard out.
(I believe it will still send the logging to /var/log/keystone/keystone.log, not sure how to actually get it to log to standard out when running manually like this).