Issue

• From the following command it is clear that "proxy-list" element of "mod-cluster" subsystem allows expressions "expressions-allowed" => true:

[standalone@localhost:9999 /] /subsystem=modcluster/mod-cluster-config=configuration:read-resource-description


            "proxy-list" => {
                "type" => STRING,
                "description" => "List of proxies, Format (hostname:port) separated with comas.",
                "expressions-allowed" => true,
                "nillable" => true,
                "min-length" => 1L,
                "max-length" => 2147483647L,
                "access-type" => "read-write",
                "storage" => "configuration",
                "restart-required" => "all-services"
            },

• Still the following kind of expression always fail to configure successfully and only '$' symbol appears in the configuration file.

[standalone@10.10.10.10:9999 /] /subsystem=modcluster/mod-cluster-config=configuration:write-attribute(name=proxy-list,value="${jboss.modcluster.proxyList:127.0.0.1:10001}")
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    }
}
[standalone@10.10.10.10:9999 /] /subsystem=modcluster/mod-cluster-config=configuration:read-attribute(name=proxy-list)
{
    "outcome" => "success",
    "result" => "$",
    "response-headers" => {"process-state" => "reload-required"}