Do not fail if software directory does not exists.

slapos/grid/SlapObject.py

@@ -183,6 +183,7 @@ class Software(object):
         os.chmod (path, 0600)
         func(path)
     try:
+      if os.path.exists(self.software_path):
         shutil.rmtree(self.software_path, onerror=retry)
     except IOError as error:
       error_string = "I/O error while removing software (%s): %s" % (self.url,