Change the default l3_agent_manager to L3NATAgent

Change the default l3_agent_manager to L3NATAgent

This is a l3-agent version of 349aa3ef3563c019f4d0a98fc30117ae0a6cade3.

The combination of the plugin without agent extension support
and L3NATAgentWithStateReport results in an error:
"AttributeError: No such RPC function 'report_state'".

This patch changes l3_agent_manager to be L3NATAgent instead of
L3NATAgentWithStateReport since all plugins do not support
L3NATAgentWithStateReport and having this as the default breaks
all current deployments that upgrade source without changing
their config files.

Fixes bug 1139726

Change-Id: Ibb5c04c89bfad741ec43a372cbf97445f7b3d76c