Add additional web application configuration feature.

This extension allows the WebappBootstrap to read context parameters from the servlet context. If no context configuration is used or the properties wasn't found, the bootstrap is executed in the default way.

The ConfigurationBuilder now also tries to interpret the given location as absolute path if the requested file was not found within the class path. This is required, otherwise an absolute path from the servlet context is only looked up in the class path and not in the local file system.

Finally this feature allows a more flexible configuration of the nexus data directory, when deployed as war file within an existing servlet container like tomcat or jetty.