fix autorequire dependency handling

- inherit explicit requirements that the user defined himself
- but _not_ implicit requirements that cdist added for autorequire

Changes:
- added new .autorequire property to CdistObject to keep track of implicit autorequire dependencies
- Emulator appends implicit requirements to this .autorequire property
- DependencyResolver preprocess these .autorequire properties before resolving normal dependencies
- refactored and documented DependencyResolver so it's clearer what happens and easier to use from tests
- update test cases to match new DependencyResolver behaviour

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>

lib/cdist/config_install.py

@@ -141,7 +141,7 @@ class ConfigInstall(object):
             self.local.type_path)
 
         dependency_resolver = resolver.DependencyResolver(objects)
-        self.log.debug(pprint.pformat(dependency_resolver.graph))
+        self.log.debug(pprint.pformat(dependency_resolver.dependencies))
 
         for cdist_object in dependency_resolver:
             self.log.debug("Run object: %s", cdist_object)