Refactor mount/mode setting for local SSD RAID

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 GoogleCloudPlatform#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.

modules/scripts/startup-script/files/setup-raid.yml

@@ -53,7 +53,7 @@
         [Unit]
         After=local-fs.target
         Before=slurmd.service
-        ConditionPathIsMountPoint=!{{ mountpoint }}
+        ConditionPathExists=!{{ array_dev }}
 
         [Service]
         Type=oneshot
@@ -70,19 +70,28 @@
       enabled: true
       daemon_reload: true
 
-  - name: Mount RAID array
-    ansible.posix.mount:
-      src: "{{ array_dev }}"
-      path: "{{ mountpoint }}"
-      fstype: "{{ fstype }}"
-      # the nofail option is critical as it enables the boot process to complete on machines
-      # that were powered off and had local SSD contents discarded; without this option
-      # VMs may fail to join the network
-      opts: discard,defaults,nofail
-      state: mounted
+  - name: Install service to mount local SSD array
+    ansible.builtin.copy:
+      dest: /etc/systemd/system/mount-localssd-raid.service
+      mode: 0644
+      content: |
+        [Unit]
+        After=local-fs.target create-localssd-raid.service
+        Before=slurmd.service
+        Wants=create-localssd-raid.service
+        ConditionPathIsMountPoint=!{{ mountpoint }}
 
-  - name: Set mount permissions
-    ansible.builtin.file:
-      path: "{{ mountpoint }}"
-      state: directory
-      mode: "{{ mode }}"
+        [Service]
+        Type=oneshot
+        ExecStart=/usr/bin/systemd-mount -t {{ fstype }} -o discard,defaults,nofail {{ array_dev }} {{ mountpoint }}
+        ExecStartPost=/usr/bin/chmod {{ mode }} {{ mountpoint }}
+
+        [Install]
+        WantedBy=slurmd.service
+
+  - name: Mount RAID array and set permissions
+    ansible.builtin.systemd:
+      name: mount-localssd-raid.service
+      state: started
+      enabled: true
+      daemon_reload: true