sethostname hook from http://docs.cloudstack.apache.org/projects/cloudstack-administration/en/4.4/templates.html#creating-a-linux-template

cs-sethostname

#!/bin/sh
# dhclient change hostname script for Ubuntu
oldhostname=$(hostname -s)
if [ $oldhostname = 'localhost' ]
then
    sleep 10 # Wait for configuration to be written to disk
    hostname=$(cat /var/lib/dhcp/dhclient.eth0.leases  |  awk ' /host-name/ { host = $3 }  END { printf host } ' | sed     's/[";]//g' )
    fqdn="$hostname.$(cat /var/lib/dhcp/dhclient.eth0.leases  |  awk ' /domain-name/ { domain = $3 }  END { printf     domain } ' | sed 's/[";]//g')"
    ip=$(cat /var/lib/dhcp/dhclient.eth0.leases  |  awk ' /fixed-address/ { lease = $2 }  END { printf lease } ' | sed     's/[";]//g')
    echo "cloudstack-hostname: Hostname _localhost_ detected. Changing hostname and adding hosts."
    printf " Hostname: $hostname \n FQDN: $fqdn \n IP: $ip"
    # Update /etc/hosts
    awk -v i="$ip" -v f="$fqdn" -v h="$hostname" "/^127/{x=1} !/^127/ && x { x=0; print i,f,h; } { print $0; }" /etc/hosts > /etc/hosts.dhcp.tmp
    mv /etc/hosts /etc/hosts.dhcp.bak
    mv /etc/hosts.dhcp.tmp /etc/hosts
    # Rename Host
    echo $hostname > /etc/hostname
    hostname -b -F /etc/hostname
    # Recreate SSH2
    export DEBIAN_FRONTEND=noninteractive
    dpkg-reconfigure openssh-server
fi
### End of Script ###