Validate that node_version is set

We removed the default value, because having a default value actually
makes no sense at all. To be helpful for any transitions, add a runtime
check that the variable is set.

Also, while we're at it, update the docs to indicate that the parameter
is required.

Change-Id: I1e18ea51d9d56561608ff241d71b63965c4f78bd

roles/ensure-nodejs/README.rst

@@ -3,4 +3,5 @@ Install NodeJS from nodesource
 **Role Variables**
 
 .. zuul:rolevar:: node_version
-   :default: 6
+
+   Required. What version of Node to install.

roles/ensure-nodejs/tasks/main.yaml

@@ -1,3 +1,7 @@
+- name: Ensure we know what version of node to install
+  assert:
+    that: node_version is defined
+
 - name: Update apt cache
   apt:
     update_cache: yes