Update config file check to use os.path.isfile.

The previous check os.path.exists also checks for directories, os.path.isfile seems to be a cleaner way to check if a file exists.

Author: donnemartin

awsshell/config.py

@@ -77,7 +77,7 @@ def _copy_template_to_config(self, template_path,
             existing config file, if it exists.
         """
         expanded_config_path = os.path.expanduser(config_path)
-        if not overwrite and os.path.exists(config_path):
+        if not overwrite and os.path.isfile(config_path):
             return
         else:
             shutil.copyfile(template_path, config_path)