dist/ami: Switch to official CentOS base image
dist/ami: Switch to official CentOS base image

We had switched our own CentOS base image since we couldn't make built AMI to
public due to base image settings, it's probably because the image provided via
AWS market place.
However, I've found an official image outside of market place, and I succeeded
making built AMI to public based on the image.
URL: https://wiki.centos.org/Cloud/AWS

Once we could able to use official image, we probably should use official one.

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1513659164-28029-1-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

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