Set webAppRootKey in web.xml to fix 404 error.

The metadata building block suddenly started 404ing. Investigation of
the logs show that another building block was setting the webapp.root
configuration before us, causing the metadata editor to throw an
exception and exit. Turns out that if you're running multiple Tomcat
apps using Spring, you must set a unique webAppRootKey or there'll be
conflicts.

From: http://stackoverflow.com/a/5019680

The webAppRootKey is a context parameter that Spring uses in a couple of
places. In this case, it's being used by the Log4jWebConfigurer. It
exposes the webapp root as a system property that can be used in log4j
configuration files, something like this:

log4j.appender.testfile.File=${webapp.root}/WEB-INF/testlog.log

You would use this if you, for some reason, wanted to locate your logs
relative to your webapp root.

The problem that you're running into is that some containers (notably
Tomcat) don't maintain a per-webapp mapping of system properties. When
you don't specify a webAppRootKey, Spring defaults it to webapp.root.
Since you're running two apps in the same container, the second app
you're trying to start up sees that the webAppRootKey is already set
(via the default), and throws an error. Otherwise, the webAppRootKey
would be set incorrectly, and you could end up with logs from one webapp
in another webapp.