Fix hostname setting for Centos.

Fix hostname setting for Centos.

This patch sets the hostname in the suts by command line
instead of using the cloud-init hostname module

This fix works for both the newest Centos 7.4-1708 and the older
Centos version in version 1.4

This patch should be merged separately from the patch
updating the Centos listmaker image to 7.4-1711.

Change-Id: Ie9f476ec82356fbd35077b74c55dd1a78e60248f
Signed-off-by: Thomas F Herbert <therbert@redhat.com>
(cherry picked from commit 86186044368819d66b68732d3e28876e7fcf3561)