Make hooks module import optional

There is no hooks module available when running the
extract_swift_flags.py locally, so making this import optional.

Change-Id: I65ebacd1e3e4cf66ece0a9fcbbc785d79278e8bb

autogenerate_config_docs/autohelp.py

@@ -36,7 +36,10 @@ except Exception:
     pass
 
 sys.path.insert(0, '.')
-from hooks import HOOKS  # noqa
+try:
+    from hooks import HOOKS  # noqa
+except ImportError:
+    pass
 
 
 EXTENSIONS = ['oslo.cache',