Use absolute path to load cassandra-driver.conf

Otherwise, cassandra config file can not load correctly using relative path.
cf:
```
JAMES-3668 Add absolute paths for extra properties file in docker image

We had issues with the relative path not working with Docker and James not being able to find the file.
Overriding to absolute path should ensure it finds it in this particular case.
```

tmail-backend/apps/distributed/src/main/conf/jvm.properties

@@ -5,7 +5,7 @@
 # my.property=whatever
 
 # Required to locate Cassandra driver configuration
-config.file=conf/cassandra-driver.conf
+config.file=/root/conf/cassandra-driver.conf
 
 # (Optional). String (size, integer + size units, example: `12 KIB`, supported units are bytes KIB MIB GIB TIB). Defaults to 100KIB.
 # This governs the threshold MimeMessageInputStreamSource relies on for storing MimeMessage content on disk.