This snitch will read the EC2 availability zone and set the DC
and RACK as follows:
If availability zone is "us-east-1d", then
DC="us-east" and RACK="1d".
If cassandra-rackdc.properties contains "dc_suffix" field then
DC will be appended with its value.
For instance if dc_suffix=_1_cassandra, then in the example above
DC=us-east_1_cassandra
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>