Fixing typo and adding example policy

Keystone Architecture in OpenStack Training Guides has
typo with cinder policy instead of nova policy filename.
Also, the policy sample was missing. Added policy sample
on privileges for creating volumes.

Change-Id: I85e2e6a3c75a98e2cb5c5e5eda3ff8647aa32e66
Closes-bug: #1326104
Merge "Fixing typo and adding example policy"