Neaten up bastion SSH proxy config

Author: m-bull

roles/image_build/defaults/main.yml

@@ -32,8 +32,13 @@ image_build_builder_group_vars:
 
 # ansible_ssh_common_args for Packer build
 image_build_ansible_ssh_common_args: >-
+    {% if image_build_ssh_bastion_host is defined %}
     '-o ProxyCommand="ssh -W %h:%p -q
     {% if image_build_ssh_bastion_private_key_file is defined %}
     -i {{ image_build_ssh_bastion_private_key_file }}
     {% endif %}
-    {{ image_build_ssh_bastion_username }}@{{ image_build_ssh_bastion_host }}"'
+    -l {{ image_build_ssh_bastion_username }}
+    {{ image_build_ssh_bastion_host }}"'
+    {% else %}
+    ""
+    {% endif %}