Save the following script to /etc/dhcp/dhclient-exit-hooks.d/sethostname, and adjust the permissions. cf. http://docs.cloudstack.apache.org/projects/cloudstack-administration/en/4.5/templates.html#system-preparation-for-linux

sethostname

#!/bin/sh
# dhclient change hostname script for Ubuntu
# ( cf. http://docs.cloudstack.apache.org/projects/cloudstack-administration/en/4.5/templates.html#system-preparation-for-linux )
oldhostname=$(hostname -s)
if [ $oldhostname = 'localhost' -o $oldhostname = '(none)' ]; then
  sleep 10 # Wait for configuration to be written to disk
  hostname=$(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\n"
  # Update /etc/hosts
  if [ ${#ip} -gt 4 ]; then
    awk -v i="$ip" -v f="$fqdn" -v h="$hostname" '/^127/{x=1} !/^127/ && x { x=0; printf("%s\t%s\t%s\n",i,f,h); } { print $0; }' /etc/hosts > /etc/hosts.dhcp.tmp
    #cp -p /etc/hosts /etc/hosts.dhcp.tmp
    #echo "$ip $fqdn $hostname" >> /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
    echo $hostname > /proc/sys/kernel/hostname
  fi
  # Recreate SSH2
  export DEBIAN_FRONTEND=noninteractive
  dpkg-reconfigure openssh-server
fi