Package WildFly applications as .war instead of .jar

This lets Servlets take advantage of session replication when
clustered and gives us a visible web deployment in the CLI and
management console of WildFly.

The .war needs to have, at minimum, a WEB-INF/web.xml and
WEB-INF/lib/foo.jar, where foo.jar is the executable jar we build for
running outside of WildFly.

See https://gist.github.com/bbrowning/34d7e4a2e11c16fef9df for an
example web.xml.
Don't require :root, it isn't valid for Immutant wars.