Role shouldn't require libvirt_vm_memory_mb and libvirt_vm_vcpus to be set when destroying the VM

[priteau]

The libvirt_vm_vcpus and libvirt_vm_memory_mb variables are required by the stackhpc.libvirt-vm role even when setting libvirt_vm_state to absent. Using the role without them fails with the following error:

FAILED! => {"msg": "'libvirt_vm_vcpus' is undefined"}

See https://review.openstack.org/#/c/606132 for context.