Allow top level playbook files to import other playbook files, added some basic

playbook structure checking.

examples/playbooks/nested_playbooks.yml

@@ -0,0 +1,26 @@
+---
+# it is possible to have top level playbook files import other playbook
+# files.  For example, a playbook called could include three
+# different playbooks, such as webservers, workers, dbservers, etc.
+#
+# Running the site playbook would run all playbooks, while individual
+# playbooks could still be run directly.  This is somewhat like
+# the tag feature and can be used in conjunction for very fine grained
+# control over what you want to target when running ansible.
+
+- name: this is a play at the top level of a file  
+  hosts: all
+  user: root
+  tasks:
+  - name: say hi
+    tags: foo
+    action: shell echo "hi..."
+
+# and this is how we include another playbook, be careful and
+# don't recurse infinitely or anything.  Note you can't use
+# any variables here.
+
+- include: intro_example.yml
+
+# and if we wanted, we can continue with more includes here,
+# or more plays inline in this file