add preprocessors function

src/risotto/image.py

@@ -121,7 +121,6 @@ class Image:
 
 
     def list_images(self):
-        print(self.builds)
         for build in self.builds:
             dependencies = [self.applications[build]['path']]
             self.calc_depends(dependencies, build)
@@ -285,7 +284,16 @@ class Image:
                 recipe[key] = value
         with open(packer_filename, 'w') as recipe_fd:
             jdump(recipe, recipe_fd, indent=2)
-        print(['packer', 'build', packer_filename])
+        preprocessors = join(tmp_directory, 'preprocessors')
+        if isfile(preprocessors):
+            proc = Popen([preprocessors],
+                         #stdout=PIPE,
+                         #stderr=PIPE,
+                         cwd=tmp_directory,
+                         )
+            proc.wait()
+            if proc.returncode:
+                raise Exception(_(f'error when executing {preprocessors}'))
         proc = Popen(['packer', 'build', packer_filename],
                      #stdout=PIPE,
                      #stderr=PIPE,