messaging_service::get_rpc_client(): fix the encryption logic
messaging_service::get_rpc_client(): fix the encryption logic

According to specification
(here https://wiki.apache.org/cassandra/InternodeEncryption)
when the internode encryption is set to `dc` the data passed between
DCs should be encrypted and similarly, when it's set to `rack`
the inter-rack traffic should encrypted.

Currently Scylla would encrypt the traffic inside a local DC in the
first case and inside the local RACK in the later one.

This patch fixes the encryption logic to follow the specification
above.

Signed-off-by: Vlad Zolotarov <976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>
Message-Id: <1452501794-23232-1-git-send-email-976c0f8d0852ed6439ad273880b400a75a7c41d0@cloudius-systems.com>

duarten/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla