Feature resolver

The resolver attribute on a feature can be set to force the use of a given resolver instead of the default resolution process. A resolver will be used to obtain the list of bundles to actually install for a given feature.

The default resolver will simply return the list of bundles provided in the feature description. The obr resolver can be installed and used instead of the standard one. In that case, the resolver will use the OBR service to determine the list of bundles to install (bundles flagged as dependency will only be used as possible candidates to solve various constraints).