chapter 3: replace remove multi and use all

Ansible nowadays has 2 automatic groups called `all` meaning all defined hosts and ` ungrouped` defining all hosts that are not part of a group.

this would mean that you can remove the `[multi]` group and replace the `[multi:vars]` with `[all:vars]` tried this out with ansible version 2.14.2 and works well.

Start by locating the Chapter 3 inventory examples that define the [multi] group and its [multi:vars] section. Check how the examples use Ansible's automatic all group, then update the relevant example so the old group is gone and its variables use [all:vars]. Verify the Chapter 3 examples remain valid with Ansible 2.14.2.

Written by the indexing model from the issue text.