AttributeError: No such RPC function 'report_state'

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

Fixes bug 1131446

Change-Id: I6415f85714b03e9bf59d597e8ec2397fedc92b02
Merge "AttributeError: No such RPC function 'report_state'"