REST-spec: Allow stashed values to be used in property names as well

Fix the nodes.info/20_transport test to use the master node, rather
than to rely on applying a regex to the whole $body

rest-api-spec/test/README.asciidoc

@@ -168,11 +168,28 @@ The last response obtained gets always stashed automatically as a string, called
 This is useful when needing to test apis that return text rather than json (e.g. cat api),
 as it allows to treat the whole body as an ordinary string field.
 
+Stashed values can be used in property names, eg:
+
+....
+  - do:
+      cluster.state: {}
+
+  - set: {master_node: master}
+
+  - do:
+      nodes.info:
+        metric: [ transport ]
+
+  - is_true: nodes.$master.transport.profiles
+....
+
+
 Note that not only expected values can be retrieved from the stashed values (as in the
 example above), but the same goes for actual values:
 
 ....
-    - match: { $body: /^.+$/ } # the returned `body` matches the provided regex
+    - match: { $body: /^.+$/ } # the returned `body` matches the provided regex if the body is text
+    - match: { $body: {} } # the returned `body` matches the JSON object if the body is JSON
 ....
 
 The stash should be reset at the beginning of each test file.

rest-api-spec/test/nodes.info/20_transport.yaml

@@ -1,9 +1,13 @@
 ---
+
 "node_info test profile is empty":
+  - do:
+      cluster.state: {}
+
+  - set: {master_node: master}
+
   - do:
       nodes.info:
         metric: [ transport ]
 
-# there is no possbility to use is_true here due to unknown node_id
-# which is part of the path, just checking for profiles in the body
-  - match: { $body: /profiles/ }
+  - is_true: nodes.$master.transport.profiles