Delete segments on BEFORE_DELETE instead of PRECOMMIT

Deleting segments results in BEFORE/PRECOMMIT/AFTER events
itself so it shouldn't performed while already inside a transaction
to delete a network.

Closes-Bug: #1675910
Change-Id: I6c8070e1dc2216ea909288ce1133321e8eda1daa

neutron/services/segments/db.py

@@ -324,6 +324,6 @@ def subscribe():
                        resources.SEGMENT, events.PRECOMMIT_CREATE)
     registry.subscribe(_delete_segments_for_network,
                        resources.NETWORK,
-                       events.PRECOMMIT_DELETE)
+                       events.BEFORE_DELETE)
 
 subscribe()