Support using images with binaries pre-installed and use for smslabs CI

[sjpb]

stackhpc.slurm_image_builder can build an image which includes nearly all binaries required for all nodes in a default slurm appliance.

This PR:

• Updates the smslabs environment to use an image from slurm_image_builder for both direct-deploy and as the base for image build, to try to reduce CI cycle time.
• Uses a cloudalchemy.node_exporter fork which copes with binaries pre-existing (see Allow preprovisioning binaries cloudalchemy/ansible-node-exporter#265). This avoids the normal "localhost download + propagate to nodes" approach taken by this role.
• Disables prometheus binary installs when it finds binaries already exist and no specific version is required.

Current deployment: vglabs-steveb-ansible-rocky85:/home/rocky/slurm-app-prebuilt