Decouple backup/site name from the name of the corresponding resource

Currently, the site names defined in the JGroups subsystem RELAY2 protocol, and the backup names defined in the Infinispan subsystem do not allow expressions, since these name are used for the corresponding resource paths. To simplify the use of cross-site replication (specifically, to allow each site to use an identical domain.xml/standalone.xml), we can support expressions if we decouple the site name from the resource name.
e.g.

<backups>
    <backup name="remote" site="${...}" strategy="SYNC"/>
</backups>

<relay site="local">
    <remote-site name="local" site-name="${...}" channel=".."/>
    <remote-site name="remote" site-name="${...}" channel=".."/>
</relay>