Enable local SSD formatting solution to set POSIX permissions

[tpdownes]

#2720 did not expose the ability to set filesystem permissions to the blueprint developer. This PR adds that functionality. Tested with

---
blueprint_name: startup-vm-instance

vars:
  project_id:  ## Set project id here
  deployment_name: testfix
  region: us-east4
  zone: us-east4-b
  bandwidth_tier: gvnic_enabled

deployment_groups:
- group: first
  modules:
  - id: network1
    source: modules/network/vpc
  - id: script
    source: modules/scripts/startup-script
    settings:
      local_ssd_filesystem:
        mountpoint: /mnt/ssd
        permissions: 1777
  - id: vm1
    source: modules/compute/vm-instance
    use:
    - network1
    - script
    settings:
      machine_type: n2d-standard-16
      disk_type: pd-balanced
      local_ssd_count: 2
      instance_image:
        project: cloud-hpc-image-public
        family: hpc-rocky-linux-8

Specifically:

1. I provisioned the blueprint above and observed the correct permissions.
2. I modified the blueprint to comment out the permissions, re-deployed and then ran systemctl start google-startup-scripts.service. The permissions changed to 0755 (the correct default)
3. I tested values that are not 3 or 4 digit octals and got terraform validation errors.

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