Remove defaults for required vars

roles/virtualization/templates/vm.xml.j2

@@ -1,7 +1,7 @@
 <domain type='kvm'>
   <name>{{ hostname }}</name>
   <memory>{{ vm_memory | int * 1024 }}</memory>
-  {% if vm_ballo | int > 0 %}<currentMemory>{{ vm_ballo | int * 1024 }}</currentMemory>{% endif %}
+  {% if vm_ballo is defined and vm_ballo | int > 0 %}<currentMemory>{{ vm_ballo | int * 1024 }}</currentMemory>{% endif %}
   <vcpu placement='static'>{{ vm_cpus }}</vcpu>
   <os>
     <type arch='x86_64' machine="pc-q35-8.0">hvm</type>
@@ -37,7 +37,7 @@
       <source file="{{ virtualization_libvirt_cloudinit_path }}"/>
       <target dev="sdb" bus="sata"/>
     </disk>
-    {% if rhel_iso | length > 0 %}
+    {% if rhel_iso is defined and rhel_iso | length > 0 %}
     <disk type="file" device="cdrom">
       <driver name="qemu" type="raw"/>
       <source file="{{ rhel_iso }}"/>