Don't fail if glance_store is not available

The autohelp glance hook can only work starting with Juno. So ignore
import errors for other versions.

Change-Id: I40570db1cc83ba651e1eb2ba47560a6dc757e326

autogenerate_config_docs/hooks.py

@@ -25,10 +25,14 @@ def keystone_config():
 
 
 def glance_store_config():
+    try:
         import glance_store
         from oslo.config import cfg
 
         glance_store.backend.register_opts(cfg.CONF)
+    except ImportError:
+        # glance_store is not available before Juno
+        pass
 
 
 HOOKS = {'keystone.common.config': keystone_config,