Refactor mount/mode setting for local SSD RAID

[tpdownes]

The local SSD RAID solution is written in Ansible which will successfully handle re-creating the RAID array and mounting it in scenarios where the VM has been re-created and the contents of local SSD have been discared. The Slurm solutions do not re-run startup scripts after the first boot using a given persistent disk. During maintenance events, the persistent disk is retained while the local SSD disks are discarded. PR #3129 addressed re-creating, formatting and mounting the RAID array but left a gap in setting the mode of the mounted directory after power off/on cycles. This PR refactors mounting and mode-setting to resolve this gap.

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[tpdownes]

Noting: PR-test-ml-a3-megagpu-slurm passed before squashing commits prior to merge.

[tpdownes]

Testing with

#!/bin/bash

set -ex
mountpoint /mnt/localssd
ls -lhd /mnt/localssd
cat /etc/enroot/enroot.conf

and

srun --container-image=alpine grep PRETTY /etc/os-release

both demonstrated end-to-end functionality of enroot on local SSD volumes on first-boot/reboot/power off-on/re-creation.