Added documentation about databags

When you don't use the developer_mode you need to have crypted databag to store the openstack password.
This patch explain all the items you need and add some example of the items schema.

Change-Id: I67d6bfecb9df2da92af064793ad6d3676f93bbc8
Closes-Bug: #1282457
Merge "Added documentation about databags"