Use hostname -s instead of hostnamectl --transient

Use hostname -s instead of hostnamectl --transient

This patch updates the deployed-server interface to use a
simple hostname -s. The previous hostnamectl --transient
can pick up extra domain name configuration in some cases
that can cause very odd hostname generation if used
with the tripleo-heat-template host file generation.

This would actually break the new undercloud t-h-t installer
in that some of the /etc/hosts entries would be invalid
(no IP address) due to substring replacements failing in
a variety of odd hostname situations. Simplifying the
hostname of deployed servers to just the short version seems
the most sensable way to avoid all this.

Change-Id: Ia7e636d021f948ea5234475cef02f666d8ce6999