Provide a default configuration file for the distributed build agent

Currently it's harder than it needs to be to configure a distributed build agent.

The continuum-buildagent.xml file contains only an empty <continuum-buildagent-configuration/> element and must be completed by the user.

Here is a minimal config file that will work, placing the data directories relative to the installation and assuming that the master is on the same server. These are the same kind of defaults that the Continuum webapp provides.

<continuum-buildagent-configuration>
  <continuumServerUrl>http://localhost:8080/continuum/master-xmlrpc</continuumServerUrl>
  <buildOutputDirectory>data/build-output-directory</buildOutputDirectory>
  <workingDirectory>data/working-directory</workingDirectory>
</continuum-buildagent-configuration>