template.json

{    
    "variables": {
       "os_username": "{{env `OS_USERNAME`}}",
       "os_password": "{{env `OS_PASSWORD`}}",
       "os_auth_url": "{{env `OS_AUTH_URL`}}",
       "os_region_name": "{{env `OS_REGION_NAME`}}",
       "os_tenant_name": "{{env `OS_TENANT_NAME`}}",
       "ssh_username": "{{env `SSH_USERNAME`}}",
       "os_domaine_name": "{{env `OS_USER_DOMAIN_NAME`}}"
       },
   
       "builders": [
           {
               "type": "openstack",
               "identity_endpoint": "{{user `os_auth_url`}}",
               "tenant_name": "{{user `os_tenant_name`}}",
               "domain_name": "{{user `os_domaine_name`}}", 
               "username": "{{user `os_username`}}",
               "password": "{{user `os_password`}}",
               "ssh_username": "{{user `ssh_username`}}",
               "region":  "{{user `os_region_name`}}",
               "image_name": "packer-image",
               "instance_name": "packer-instance",
               "availability_zone": "eu-west-0a",
               "use_blockstorage_volume": false,
               "source_image": "the ID of the public image",
               "flavor": "c2.xlarge",
               "networks": ["the ID of the network"],
               "floating_ip": "the ID of the elastic IP"
           }
       ],
       "provisioners": [
           {
               "script": "setup-vm.sh",
               "type": "shell"
           }
       ]
   }