Fix issues when using GenericCloud image

[sjpb]

• Upgrade to python3.9 and latest pip for Open Ondemand websockify venv to fix error caused by pip changes:

  [openondemand : Install websockify package in venv] ... ModuleNotFoundError: No module named 'setuptools_rust'

  Note the venv path is changed to avoid any potential upgrade issues.

• Ensure slurm user is pre-installed on all nodes to avoid potential for uid/gid clash.

• Provides new image openhpc-230922-0940-434e190f, requiring 10GB root disk.

• Arcus packer build now uses volume-backed instances to achieve this.

[sjpb]

Tested above commit works (including OOD remote desktop) on GenericCloud image.

[sjpb]

Cancelled CI. Fat image build running here: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/6272655138/job/17034592810

[sjpb]

Checked remote desktop and slurm jobs dashboard works OK at above commit.