fence_sbd: Update fence_sbd.xml

Author: liangxin1300

tests/data/metadata/fence_sbd.xml

@@ -8,7 +8,7 @@
 		<content type="string" default="reboot"  />
 		<shortdesc lang="en">Fencing action</shortdesc>
 	</parameter>
-	<parameter name="devices" unique="0" required="1">
+	<parameter name="devices" unique="0" required="0">
 		<getopt mixed="--devices=[device_a,device_b]" />
 		<content type="string"  />
 		<shortdesc lang="en">SBD Device</shortdesc>