Starting to add additional unit tests for VariableManager

Required some rewiring in inventory code to make sure we're using
the DataLoader class for some data file operations, which makes mocking
them much easier.

Also identified two corner cases not currently handled by the code, related
to inventory variable sources and which one "wins". Also noticed we weren't
properly merging variables from multiple group/host_var file locations
(inventory directory vs. playbook directory locations) so fixed as well.

test/units/mock/loader.py

@@ -57,6 +57,10 @@ class DictDataLoader(DataLoader):
     def list_directory(self, path):
         return [x for x in self._known_directories]
 
+    def is_executable(self, path):
+        # FIXME: figure out a way to make paths return true for this
+        return False
+
     def _add_known_directory(self, directory):
         if directory not in self._known_directories:
             self._known_directories.append(directory)