"logging.config" property declared in "application.yml" not taken when deployed to Tomcat

### Task List

- [x] Steps to reproduce provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

1. Crate a simple application with Grails 3.3.10
2. Define and set the property "logging / config" in "application.yml" as described here: https://docs.grails.org/4.0.0/guide/single.html#logging (see point 4.1.3.3)
3. Deploy to Tomcat

### Expected Behaviour

The application reads the "logback.groovy" from the defined path and create the log files

### Actual Behaviour

The application doesn't create the log files.
The only way to externalize seems to define a system property like "-Dlogging.config=/Users/me/config/logback.groovy"

### Environment Information

- **Operating System**: Windows server 2012 R2
- **Grails Version:** 3.3.10
- **JDK Version:** Oracle 1.8 update 201
- **Container Version (If Applicable):** Tomcat 8.0.37

### Example Application

https://github.com/leleFc/TestLog

Start with the linked TestLog example application and reproduce the deployment on Tomcat using the logging/config property in application.yml. Compare that behavior with the -Dlogging.config system property and trace how logback.groovy is located. Done means the configured external logback.groovy is read when deployed to Tomcat and the expected log files are created.

Written by the indexing model from the issue text.