ubuntu 12.04装好openstack之后，安装成功，终端打出如下信息：
Horizon is now available at http://192.168.0.2/
Keystone is serving at http://192.168.0.2:5000/v2.0/
Examples on using novaclient command line is in exercise.sh
The default users are: admin and demo
The password: wertyu
This is your host ip: 192.168.0.2
stack.sh completed in 7199 seconds.
当时进http://192.168.0.2/是可以登录进去的。重启之后登不上，提示“An error occurred authenticating. Please try again later.”
解决方法：
运行~/devstack目录下的rejoin-stack.sh，然后就能登录了。
原因：
* Horizon use keystone to authenticate.
* devstack run all the service in a screen session.
* When you reboot, this screen session is stopped and all the services with it.
* Running "rejoin-stack.sh" relaunch the screen session and the associated
* services (notably keystone).
* Without this script, no keystone => no auth.
* So run the script ./rejoin-stack.sh and login in horizon after.
参考：http://fosshelp.blogspot.com/2013/01/how-to-fix-openstack-login-error-error.html