dist/common/scripts/scylla_setup: print message when EC2 instance is optimized for Scylla
dist/common/scripts/scylla_setup: print message when EC2 instance is optimized for Scylla

Currently scylla_ec2_check exits silently when EC2 instance is optimized
for Scylla, it's not clear a result of the check, need to output
message.

Note that this change effects AMI login prompt too.

Fixes #3655

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <20180808024256.9601-1-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla