ofagent: move main module from ryu repository

the recent oslo.messaging transition broke ofagent.
this commit fixes the regression.

historically the main module of ofagent has been maintained
in ryu's repository.  this commit moves it into neutron's
repository and add common_config.init() call which is necesary
for the new rpc code.

Change-Id: Icf215b20cd84ef44b51439e55b1815d72041ba3f