datavol: Support creating btrfs subvolumes

Set the `btrfs_subvolumes` variable to an array of objects with `name`
and `device` properties to create btrfs subvolumes.

datavol.yml

@@ -29,6 +29,21 @@
     tags:
     - mkfs
 
+  - name: ensure btrfs subvolumes exist
+    community.general.btrfs_subvolume:
+      name: '{{ item.name }}'
+      filesystem_device: '{{ item.device }}'
+      automount: true
+    loop: '{{ btrfs_subvolumes|d([]) }}'
+    tags:
+    - btrfs
+
+  - name: ensure mount point exists
+    file:
+      path: '{{ item.mountpoint }}'
+      state: directory
+    loop: '{{ data_volumes|d([]) }}'
+
   - name: ensure data volume is mounted
     mount:
       path: '{{ item.mountpoint }}'