Support file extension in playbook path

This allows us to specify the full playbook name, with file extension.
A documentation update is included in a following patch, as well as
a deprecation notice for the old form.

Change-Id: I19a3ec50e473f717a7e5e2824f702ad4e6acab02

zuul/executor/server.py

@@ -936,7 +936,7 @@ class AnsibleJob(object):
                     "non-trusted repo." % (entry, path))
 
     def findPlaybook(self, path, required=False, trusted=False):
-        for ext in ['.yaml', '.yml']:
+        for ext in ['', '.yaml', '.yml']:
             fn = path + ext
             if os.path.exists(fn):
                 if not trusted: