Use a variable for the lvm volumn group name.

The Debian installer defaulted to `lvm` and we're unlikely to
change it, but using a variable makes it less likely we'll
make errors if we do change it in the future.

mf4.yaml

@@ -6,43 +6,47 @@
   hosts: mf4.xiph.org
   remote_user: root
 
+    # Debian installer defaults to `lvm` for the volume group name.
+    vg: lvm
+
+
   tasks:
     - name: lvm volume group on RAID
       lvg:
         # Debian installer defaults to `lvm` for the volume group name.
-        vg: lvm
+        vg: "{{ vg }}"
         pvs: /dev/sda3
 
     - name: lvm root volume
       lvol:
-        vg: lvm
+        vg: "{{ vg }}"
         lv: root
         size: 64G
 
     - name: lvm swap volume
       lvol:
-        vg: lvm
+        vg: "{{ vg }}"
         lv: swap
         size: 12G
     - name: mkswap
       filesystem:
         fstype: swap
-        dev: /dev/mapper/lvm-swap
+        dev: /dev/mapper/{{ vg }}-swap
 
     - name: lvm home volume
       lvol:
-        vg: lvm
+        vg: "{{ vg }}"
         lv: home
         size: 1T
     - name: home filesystem
       filesystem:
         fstype: ext4
-        dev: /dev/mapper/lvm-home
+        dev: /dev/mapper/{{ vg }}-home
         opts: -L mf4-home
     - name: home volume mountpoint
       mount:
         state: mounted
-        src: /dev/mapper/lvm-home
+        src: /dev/mapper/{{ vg }}-home
         path: /home
         fstype: ext4
         opts: defaults,errors=remount-ro