Add xtrabackup group explicitly

This patch ensures that "xtrabackup" group is created prior to creating
"xtrabackup" user

Change-Id: I7072b70e314c94d709ba5a9e2c8760a3a91a158d
Related-PROD: PROD-23546
diff --git a/xtrabackup/server.sls b/xtrabackup/server.sls
index 745e9a3..d8f6971 100644
--- a/xtrabackup/server.sls
+++ b/xtrabackup/server.sls
@@ -22,6 +22,13 @@
   - system: true
   - home: {{ server.backup_dir }}
 
+xtrabackup_group:
+  group.present:
+  - name: xtrabackup
+  - system: true
+  - require_in:
+    - user: xtrabackup_user
+
 {{ server.backup_dir }}/full:
   file.directory:
   - mode: 755