Check existence of node types referenced in specs

ansible/schedule.yml

@@ -1,4 +1,10 @@
 ---
+- name: Check that all specified node types exist
+  fail:
+    msg: The non-existent node type {{ item }} was specified in 'specs'.
+  when: item not in node_types
+  loop: "{{ specs.keys() }}"
+
 # Creates a dict mapping each hypervisor's hostname to its hostvars, to be used
 # during scheduling.
 - name: Collect hypervisor hostvars