Don't re-install cloud-init for centos7 images

The centos7 images come with the base version of cloud-init already
installed.

---
$ virt-inspector CentOS-7-x86_64-GenericCloud-GA-7.0.1406_01.qcow2
...
      <application>
        <name>cloud-init</name>
        <version>0.7.5</version>
        <release>10.el7.centos.1</release>
        <arch>x86_64</arch>
      </application>
...
---

I think this is a hang-over from early rhel7 images that didn't have
it.

Change-Id: Idf488942222d9e6bca4d66f04f5af42e6707944d
Merge "Don't re-install cloud-init for centos7 images"