Fix delete_sso_server function

+ INDEX should be an attribute
+ Enforce index to be a string

Author: ExaneServerTeam

hpilo.py

@@ -910,7 +910,8 @@ def delete_federation_group(self, group_name):
 
     def delete_sso_server(self, index):
         """Delete an SSO server by index"""
-        return self._control_tag('SSO_INFO', 'DELETE_SERVER', index)
+        return self._control_tag('SSO_INFO', 'DELETE_SERVER',
+                                 attrib={'INDEX': str(index)})
 
     def delete_user(self, user_login):
         """Delete the specified user from the ilo"""