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.

Previously we assumed the shell on the target systems were 'sh'-
compliant when formatting environment variables preceding command
strings. This patch corrects that by basing the target shell type
on the DEFAULT_EXECUTABLE setting, which can be overridden on a
per-host basis using the inventory variable 'ansible_shell_type'.

Fixes #7352

rules are avaialble at https://en.wikipedia.org/wiki/Wikipedia:AutoWikiBrowser/Typos

- extra 'the' in intro_inventory
- capitalization in playbooks_roles

Reference #7102

Merges the functionality of the original jinja2 random filter with
the one we provide.

Fixes #7113

Moved a sentence. Sentence refers to script, "this particular script" before it has been introduced. Moved sentence to after intro as it confused me. I though I'd missed something..

Document that roles_path can support multiple colon-separated paths

This group is useful when using multiple inventory sources.