Added the merge logic for the advertised information

This patch adds the merging logic of the advertised information of the
peers and the routes retrieved by the RPC calls in dr_agent.py.

Change-Id: I29875b3bfa950e4310f8fc4a6a7a10b938c22c2b
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>