A code sample was not being rendered as a code block. This adds the preceding '::' reStructuredText markup.

Point to current URLs, fix a Ansible/Tower reference.

This is different wording for using the PPA to install.  I also added the installation of apt-add-repository package which isn't always installed.

(I'm a newbie but while running these I wished whatever style was being used on the docs pages would have let me  cut and paste the commands without having to edit out the $ characters manually....)

As a relatively-new user to Ansible, the Nested Loop example made me think it was looping through both lists together (`alice` would go with `clientdb`, `bob` with `employeedb`) and not actually nested. I realize now I was just being stupid, but I feel making the example lists different lengths would prevent such confusion in the future.

Changed unit tests to unit test for the singular playbook...

Typo - State should have been stage.