Enable X11 forwarding for devstack

This PR enables X11 forwarding for the devstack Vagrant image.  This should make it *much* easier to develop and debug acceptance tests.

When enabled, GUI applications started on the Vagrant instance appear in a window on the host machine:
![image](https://f.cloud.github.com/assets/2948394/1783713/3d8c936a-68c7-11e3-9cd5-ad87e48335e6.png)

To enable it on Mac OS X:
1) Install [XQuartz](http://xquartz.macosforge.org/landing/)
2) `export VAGRANT_X11=1`
3) `vagrant up` (if not started) or `vagrant reload` (if already running).  If using an existing instance, use `vagrant provision` to update the configuration.
4) `vagrant ssh` into the instance, then start a GUI application such as `xclock` or `firefox`

Suggested reviewers: @feanil @jzoldak