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`}}",
        "instance_name": "{{env `INSTANCE_NAME`}}",
        "source_id": "{{env `SOURCE`}}",
        "os_flavor_id": "{{env `FLAVOR`}}",
        "os_network_id": "{{env `NETWORK_ID`}}",
        "os_floating_id": "{{env `FLOATING_ID`}}"
       },
       "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": "{{user `INSTANCE_NAME`}}",
            "source_image": "{{user `source_id`}}",
            "availability_zone": "xxx",
            "use_blockstorage_volume": false,
            "flavor": "{{user `os_flavor_id`}}",
            "networks": ["{{user `os_network_id`}}"],
            "floating_ip": "{{user `os_floating_id`}}"
           }
       ],
       "provisioners": [
           {
            "type": "file",
            "source": "local-path/file",
            "destination": "destination-path/file"
           },
           {
            "script": "setup.sh",
            "type": "shell"
           },
           {
            "type": "shell",
            "inline": [
                "sudo apt-get update",
                "sudo apt-get upgrade"
               ]
           }
       ]
   }