Increasing naming conventions consistency and expliciteness for more clarity.

Inventory file variable names:
The external resources for Ansible are used in the tutorial to instruct the new user in the making of their inventory file. However, this naming convention is not maintained in the rest of the tutorial. Namely, in the "Installing Galaxy" step 3, the user is led to name its hosts [webserver], whereas the rest of the tutorial uses [galaxyservers]. Having a ready-made inventory file for the user to copy would be useful, hopefully along with the other config files in a repo as mentioned in my [other post](https://github.com/galaxyproject/training-material/issues/4499).

Playbook name:
Changing the name of the playbook from galaxy.yml to galaxy-playbook.yml would increase clarity.