Allow EnvConfig to use the values of other env variables

It turns out that kubernetes will expose useful environment variables
in config scripts, but they need to be accessed. This allows one to
use `$VAR` as a config value to make the `EnvConfig` look up that
value and return it.

Author: shs96c

java/server/src/org/openqa/selenium/grid/config/EnvConfig.java

@@ -42,6 +42,15 @@ public Optional<List<String>> getAll(String section, String option) {
       .replace("-", "_")
       .replace(".", "_");
 
-    return Optional.ofNullable(System.getenv().get(key)).map(ImmutableList::of);
+    String value = System.getenv().get(key);
+    if (value == null) {
+      return Optional.empty();
+    }
+
+    if (value.startsWith("$")) {
+      value = System.getenv(value.substring(1));
+    }
+
+    return Optional.ofNullable(value).map(ImmutableList::of);
   }
 }