Add the ability to specify context.xml files for each additional Webapp being deployed

Being able to deploy multiple webapps in an embedded container is fantastic, but currently they all inherit the same context file, which means that you can end up with namespace issues, and perhaps requiring context params (such as crosscontext) which you would rather not set on all the webapps.

As there is now good support for multi-war configurations, it'd be good to extend this to support separate context files for each war.

ie, I'd hope to do something like:

<webapps>
<webapp>
<groupId>portals</groupId>
<artifactId>portal-container</artifactId>
<version>1.9</version>
<contextPath>/container</contextPath>
<contextFile>${basedir}/portal-container-context.xml</contextFile>
</webapp>
</webapps>