Add ability to control the size of VM extradisks

Define new extradisks_size variable that is used when creating VM
"extradisks". The default value (1G) is the previously hard-coded value.

Change-Id: I440c8ce4b560a2727742b0aa051e4c8c29887026

roles/libvirt/defaults/main.yml

@@ -51,3 +51,7 @@ extradisks_list:
   - vdb
   - vdc
   - vdd
+
+# size of the disks to create when using extradisks
+extradisks_size: 1G
+

roles/libvirt/setup/overcloud/tasks/main.yml

@@ -43,7 +43,7 @@
     # These are sparse files, not using space if unused
     - name: Create additional blockdevice for objectstorage nodes
       command: >
-        dd if=/dev/zero of={{ libvirt_volume_path }}/{{ item[0].name }}_{{ item[1] }}.img bs=1 count=0 seek=1G
+        dd if=/dev/zero of={{ libvirt_volume_path }}/{{ item[0].name }}_{{ item[1] }}.img bs=1 count=0 seek={{ extradisks_size }}
       when: "{{ flavors[item[0].flavor].extradisks | default(false) }}"
       with_nested:
         - "{{ overcloud_nodes }}"