Allow <cargo> task to load configuration from external property file.

The current implementation of the ANT <cargo> task has the disadvantage that the ANT script
must still be aware which J2EE container is used (WebLogic, Resin, ...), because some J2EE-
specific properties have to be set (e.g. cargo.weblogic.server for WebLogic).

This is illustrated with build-cargo-call-current.xml (using container.properties).

A better solution would perhaps be to add an additional attribute propertiesFile or similar
on the <cargo> task that reads an external properties file from which the standard cargo
attributes all starting with 'cargo.' can be loaded as well. This is illustrated with
build-cargo-call-wishlist.xml.

The only problem that remains is to chose an overriding mechanism, e.g. when the
following is defined:

<cargo propertiesFile="container.properties" containerId="tomcat5x" ...>
...
</cargo>

and when container.properties contains the property 'cargo.containerId=weblogic8x'.
I would suggest in this case that the value of the properties value would be used
instead of tomcat5x.