[BZ 873471] Added a testcase for loading properties from URL

+ a bit of test refactoring.
[BZ 873471] Support loading props from destination-relative paths.

This is done through a custom rhq:property task that extends Ant's
default property task and adds the new "relativeToDeployDir" boolean
attribute.

The properties (both "property" and "rhq:property") are handled in the
bundle task alongside the "rhq:input-property".