Added docstrings to get_peers and get_advertisenetworks

This patch adds the docstrings to DRAgentPluginApi#get_peers and
DRAgentPluginApi#get_advertisenetworks. The both methods return the
dictionaries but it was not explicit what kind of data would be
returned. So I added them to the both methods.

Change-Id: I4f9cb05a40f024b12ad40051bbb26e7068891796
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
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>