treefile: Support variable substitution in metadata

I want to stop abusing the `rojig` key in the CoreOS treefiles. But
before we can switch over to `metadata`, we need variable substitution
supported there too.

Note the `metadata` key can have arbitrarily nested values. For now, we
just implement variable substitution for the first level of values since
that's all we need.

docs/treefile.md

@@ -26,6 +26,7 @@ It supports the following parameters:
  * `metadata`: Mapping of strings to values, optional.  This can be used
    for other tools to insert arbitrary metadata into the treefile which
    they parse later, for example via `rpm-ostree compose tree --print-metadata-json`.
+   All (top-level) objects of type string support variable substitution.
 
  * `gpg-key` (or `gpg_key`): string, optional: Key ID for GPG signing; the
    secret key must be in the home directory of the building user.  Defaults to

rust/src/treefile.rs

@@ -2855,6 +2855,13 @@ impl TreeComposeConfig {
             substitute_string(&substvars, &mut rojig.summary)?;
             substitute_string_option(&substvars, &mut rojig.description)?;
         }
+        if let Some(ref mut metadata) = self.base.metadata {
+            for val in metadata.values_mut() {
+                if let serde_json::Value::String(ref mut s) = val {
+                    substitute_string(&substvars, s)?;
+                }
+            }
+        }
         if let Some(ref mut add_commit_metadata) = self.base.add_commit_metadata {
             for val in add_commit_metadata.values_mut() {
                 if let serde_json::Value::String(ref mut s) = val {
@@ -3794,6 +3801,7 @@ conditional-include:
                 nested: table
             metadata:
                 foo: baz
+                bar: substituted ${mystr_override}
                 deeper:
                   answer: 9001
             etc-group-members:
@@ -3836,6 +3844,10 @@ conditional-include:
         );
         let data = tf.base.metadata.as_ref().unwrap();
         assert_eq!(data.get("foo").unwrap().as_str().unwrap(), "baz");
+        assert_eq!(
+            data.get("bar").unwrap().as_str().unwrap(),
+            "substituted overridden"
+        );
         assert_eq!(data.get("name").unwrap().as_str().unwrap(), "fedora-coreos");
         let deeper = data.get("deeper").unwrap().as_object().unwrap();
         assert_eq!(deeper.get("answer").unwrap().as_i64().unwrap(), 9001);