[PATCH] Add ability to specify context.xml file in plugin configuration

The context.xml file for a web application may require configuration specific to the deployment environment of the webapp, and consequently should be able to be treated like the web.xml file in allowing the location of the context.xml file to be used in the webapp to be specified by a plugin configuration parameter. This patch basically duplicates the replacement functionality provided for the web.xml file for META-INF/context.xml.

This patch also provides a minor bugfix to ensure that values of the webXml and contextXml parameters do not accept whitespace as valid values.