Ansible version 2.8.0 yields these type of deprecation warnings:
```
TASK [HanXHX.mysql : APT_KEY | Install Percona key] ****************************
[DEPRECATION WARNING]: evaluating mariadb_use_percona_apt as a bare variable,
this behaviour will go away and you might need to add |bool to the expression
in the future. Also see CONDITIONAL_BARE_VARS configuration toggle.. This
feature will be removed in version 2.12. Deprecation warnings can be disabled
by setting deprecation_warnings=False in ansible.cfg.
```
- introduces a variable certbot_from_git
- adds install from package tasks
- makes a variable called certbot_script available (to handle certbot vs certbot-auto naming)
- fixes#18
this is a preparation for installing certbot from package management
additionally, it might be useful when further tasks are added, e.g. for initial certificate retrieval
