Catch sqlalchemy.exc.InvalidRequestError exception

Running update of keystone produces the following exception which
teminates processing. This enables processing to continue, like
capture of other types of errors.
* Table 'federation_protocol' is already defined for this MetaData
  instance.  Specify 'extend_existing=True' to redefine options and
  columns on an existing Table object.

Cater for projects that do not use sqlalchemy, e.g. swift.

Change-Id: I87df4bb784f94b7194374b2448e4c34beadb923f

autogenerate_config_docs/autohelp.py

@@ -30,6 +30,11 @@ import sys
 import jinja2
 import stevedore
 
+try:
+    from sqlalchemy import exc
+except Exception:
+    pass
+
 sys.path.insert(0, '.')
 from hooks import HOOKS  # noqa
 
@@ -108,6 +113,10 @@ def import_modules(repo_location, package_name, verbose=0):
                     if verbose >= 2:
                         print(e)
                     continue
+                except exc.InvalidRequestError as e:
+                    if verbose >= 2:
+                        print(e)
+                    continue
                 except Exception as e:
                     print("Impossible to import module %s" % modname)
                     raise e