Support expressions for "site" attribute inside "backup" element in infinispan subsystem

Assume I have this in standalone.xml in infinispan subsystem:

                <distributed-cache name="foo">                    
		    <backups>
                        <backup site="${remote.site}" failure-policy="${backup.policy}" strategy="SYNC" enabled="true"/>
                    </backups>
                </distributed-cache>

and I run server with:

./standalone.sh -Dremote.site=mysite -Dbackup.policy=FAIL

I can see that "backup.policy" expression is correctly resolved, but "remote.site" is not. Indeed BackupResourceDefinition class, enum "Attribute" doesn't contain SITE attribute.

It will be nice if expressions are supported for "site" attribute too, so that I can have single ZIP distribution and use it for simulate testing with more sites and just use different system properties to simulate different site.

BTV. JGroups subsystem already supports the expressions inside "relay" element.