Script to create the guest image

This script download the latest CentOS cloud image and modify it to meet
the test requirement, including remove cloud related script, setup
the ssh keys etc.

It derives from yardstick project, which currently only provide support
to Ubuntu image. If they support CentOS image, we can possibly switch to
them.

Change-Id: I9482936615a28da696ff8f51248a62b13e5677f4
Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>