Added compatible with Ubuntu 20.04 LTS

tested with Ubuntu Server 20.04 LTS

Scaling: add new haproxy balancer node.

Add playbook "add_balancer.yml".

See more in README.md file -> "Scaling: add new haproxy balancer node".

keepalived task (added parameters, haproxy_check script).

* added parameters "enable_script_security" and "script_user root" into keepalived.conf;
https://www.keepalived.org/manpage.html

* haproxy_check vrrp script moved into /usr/libexec/keepalived directory;

* Restart the keepalived service only during the initial installation or after modification of the configuration file or haproxy_check script.
Do not restart the service when re-executing this playbook.

Update haproxy.yml

* Install lua and haproxy only if the source files has been downloaded.
Do not attempt to install haproxy packages when you re-run this book.
* Restart the haproxy service only during the initial installation or modification of the configuration file.
Do not restart the service when re-executing this playbook.
* Updated tags.