Prevent state failing if group exists

It is possible that while creating user linux will
try to create group with the same name and if it exist - the state
will fail hence explicitly adding user to group members

Change-Id: I3fe2e3a6e16ac5009bc9efc58a3c279f30349cbd
Related-PROD: PROD-23546
diff --git a/xtrabackup/server.sls b/xtrabackup/server.sls
index d8f6971..75a9b97 100644
--- a/xtrabackup/server.sls
+++ b/xtrabackup/server.sls
@@ -21,6 +21,8 @@
   - name: xtrabackup
   - system: true
   - home: {{ server.backup_dir }}
+  - groups:
+    - xtrabackup
 
 xtrabackup_group:
   group.present: