Remove update lockmode from compute_node_get_by_host

Fixes bug #948066

This commit removes the usage of an update cursor so postgres
doesn't raise the error below
SELECT FOR UPDATE/SHARE cannot be applied to the nullable side of
an outer join

Change-Id: Ia1ede22df38d5f6a24372e0ad8ec25151ad195c7

nova/db/sqlalchemy/api.py

@@ -518,22 +518,21 @@ def compute_node_update(context, compute_id, values, auto_adjust):
         compute_ref.save(session=session)
 
 
-# Note: these operations use with_lockmode() ... so this will only work
-# reliably with engines that support row-level locking
-# (postgres, mysql+innodb and above).
-
 def compute_node_get_by_host(context, host):
     """Get all capacity entries for the given host."""
     session = get_session()
     with session.begin():
         node = session.query(models.ComputeNode).\
                              options(joinedload('service')).\
                              filter(models.Service.host == host).\
-                             filter_by(deleted=False).\
-                             with_lockmode('update')
+                             filter_by(deleted=False)
         return node.first()
 
 
+# Note: these operations use with_lockmode() ... so this will only work
+# reliably with engines that support row-level locking
+# (postgres, mysql+innodb and above).
+
 def compute_node_capacity_find(context, minimum_ram_mb, minimum_disk_gb):
     """Get all enabled hosts with enough ram and disk."""
     session = get_session()