Set DB max_overflow to match thread pool size

Set max_overflow config option of database to match the number of
workers that can be spawned simultaneously. The maximum number of RPC
workers that can be spawned is determined by executor_thread_pool_size
config option.

This ensures there won't be lack of DB sessions when many worker
threads/eventlets are spawned when the engine is under load.

The DB connections that are part of overflow bucket are closed
automatically when they are not in use, so we won't run out of
'lsof' limit.

Change-Id: I512da266e7d950a4590b96dadfd283cebb51d823
Related-Prod: PROD-31284
diff --git a/metadata/service/control/single.yml b/metadata/service/control/single.yml
index 448d6fe..9275ed3 100644
--- a/metadata/service/control/single.yml
+++ b/metadata/service/control/single.yml
@@ -32,6 +32,7 @@
         name: neutron
         user: neutron
         password: ${_param:mysql_neutron_password}
+        max_overflow: 70
       identity:
         engine: keystone
         region: RegionOne
@@ -59,6 +60,7 @@
         user: openstack
         password: ${_param:rabbitmq_openstack_password}
         virtual_host: '/openstack'
+        executor_thread_pool_size: 70
       compute:
         host: ${_param:single_address}
         region: RegionOne