Fixes image creation script to work on CentOS

This patch makes a small fix to the diskimage-create.sh script,
  which allows this script to run on a CentOS 6.4/6.5 machine.
The diskimage-builder project appears to depend upon the
  python-argparse package, but in many cases this is not
  installed by default on CentOS.
This patch resolves this problem by installing the EPEL yum
  repo, as well as the python-argparse, when the CentOS
  platform is detected by the script.

Closes-Bug: #1315140

Change-Id: I51c266337b10b2dd75bf1076a409115dea979226
Merge "Fixes image creation script to work on CentOS"