Monkey patch threading module as early as possible

In oslo.messaging, local thread store is used to pass RPC request
context [1]. If we try to import oslo.messaging before monkey patching
threading library, it attempts to access unpatched storage and fails
with AttributeError.

[1]: oslo/messaging/localcontext.py#L26

blueprint oslo-messaging

Change-Id: Ied7302fcb1d3e14428540e39e3db704550027890

neutron/agent/l3_agent.py

@@ -14,6 +14,8 @@
 #
 
 import eventlet
+eventlet.monkey_patch()
+
 import netaddr
 from oslo.config import cfg
 
@@ -967,7 +969,6 @@ class L3NATAgentWithStateReport(L3NATAgent):
 
 
 def main(manager='neutron.agent.l3_agent.L3NATAgentWithStateReport'):
-    eventlet.monkey_patch()
     conf = cfg.CONF
     conf.register_opts(L3NATAgent.OPTS)
     config.register_interface_driver_opts_helper(conf)