Support json fields defined as Layout in NLog config.

Support json fields defined as Layout in NLog config.
```
<target xsi:type="RabbitMQ" useJSON="true" ...>
  <field name="threadid" layout="${threadid}"/>
  <field name="instance" layout="Node-12"/>
</target>
```
Configuration defined fields are overridden if same field occur as a LogEvent property.