ovs: remove several unneeded object attributes from setup_rpc()

ovs: remove several unneeded object attributes from setup_rpc()

It's currently not clear what the interactions and interdependencies
between existing attributes set inside setup_rpc() and other code are.
This complicates its perception. So reduce the complexity a bit by using
local variables instead of object attributes where applies; and moving
agent_id calculation out of setup_rpc() since it's not related to AMQP.

Change-Id: I6ad156a7dd686ab2a2349704a34385db1ee9681c