Implement workaround for packer ssh

The new Ubuntu Server installer starts an SSH server.
The credentials are installer:<random_pw>
Packer wrongfully tries to connect to this SSH, thinking the VM is ready for further provisioning steps - which it is NOT.

Thanks to @JulyIghor we found a workaround.
We simply change the port packer expects the ssh server to run at to something else AND during cloud-init late_commands we override the servers port accordingly. That way once the cloud-init finishes and reboots the VM the ssh server will run at the new port - now packer picks up on that and continues provisiong as we are used to.

As a last step durng provision, we remove the conf file, essentially resettign the ssh server port back to default 22.

@SwampDragons:
hashicorp/packer#9115
Please check on the logic behind communicator setting "pause_before_connecting".
That setting actually does still try to connect ONCE and then waits, instead of waiting for the specified duration and then and only then trying to connect. Thanks!

ubuntu2004/box-config.json

@@ -22,6 +22,15 @@
       "type": "shell",
       "execute_command": "echo 'vagrant' | {{.Vars}} sudo -S -E bash '{{.Path}}'",
       "script": "scripts/cleanup.sh"
+    },
+    {
+      "type": "shell",
+      "execute_command": "echo 'vagrant' | {{.Vars}} sudo -S -E bash '{{.Path}}'",
+      "inline_shebang": "#!/bin/bash -e",
+      "inline": [
+        "echo 'Resetting SSH port to default!'",
+        "rm /etc/ssh/sshd_config.d/packer-init.conf"
+      ]
     }
   ],
   "builders": [
@@ -45,7 +54,7 @@
       "iso_checksum": "caf3fd69c77c439f162e2ba6040e9c320c4ff0d69aad1340a514319a9264df9f",
       "ssh_username": "vagrant",
       "ssh_password": "vagrant",
-      "ssh_port": 22,
+      "ssh_port": 2222,
       "ssh_wait_timeout": "1800s",
       "shutdown_command": "echo 'vagrant'|sudo -S shutdown -P now",
       "guest_additions_path": "VBoxGuestAdditions_{{.Version}}.iso",

ubuntu2004/http/user-data

@@ -41,3 +41,4 @@ autoinstall:
   late-commands:
     - sed -i 's/^#*\(send dhcp-client-identifier\).*$/\1 = hardware;/' /target/etc/dhcp/dhclient.conf
     - 'sed -i "s/dhcp4: true/&\n      dhcp-identifier: mac/" /target/etc/netplan/00-installer-config.yaml'
+    - 'echo "Port 2222" > /target/etc/ssh/sshd_config.d/packer-init.conf'