Provide write-attribute handlers for JAXR config attributes

[standalone@localhost:9999 /] /subsystem=jaxr:write-attribute(name=drop-on-stop,value=true)
{
    "outcome" => "failed",
    "failure-description" => "JBAS014749: Operation handler failed: JBAS014666: Duplicate resource [(\"subsystem\" => \"jaxr\")]",
    "rolled-back" => true
}