MND-6: Update Cassandra port used by Midolman

This patch for the Reference Architecture document updates the
Cassandra port used by Midolman from the previously used Thrift
client port (9160) to the currently used CQL native transport
port (9042).

See also: https://midonet.atlassian.net/browse/MND-6

Change-Id: I1de49ba608342829ea90c4e043827faf459304ee
Signed-off-by: Jan Hilberath <jan@midokura.com>