Custom log handler properties should support expressions

The configuration of a custom-handler for the logging subsystem in standalone.xml doesn't support expressions in the value attribute of its properties/property elements. Those would be very useful, e.g. for OpenShift.

Example ([...] denote omissions for clarity):

<custom-handler [...]>
  [...]
  <properties>
    <property name="myProp" value="${env.MY_PROP_VALUE}"/>
  </properties>
</custom-handler>

TIA