Fix FELIX-3932 - Allow dependency filters to get context-source variables

Dependency filters can now have variables written like ${var}. These placeholders are replaced by values found in context-sources. System properties and instance configurations are used as context sources. Other sources can be exposed as OSGi services. 



git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1481356 13f79535-47bb-0310-9956-ffa450edef68