Introduce systemd unit restart behavior

[hwo-wd]

Introduce a flag variable that allows to setup systemd restart behaviour, and additional variables to fine-tune it:

Variable	Description	Default
keycloak_service_restart_on_failure	systemd restart-on-failure behavior activation	True
keycloak_service_startlimitintervalsec	systemd StartLimitIntervalSec	300 if keycloak_service_restart_on_failure else ``
keycloak_service_startlimitburst	systemd StartLimitBurst	5 if keycloak_service_restart_on_failure else ``
keycloak_service_restartsec	systemd RestartSec	10s if keycloak_service_restart_on_failure else ``

Fix #80

[guidograzioli]

Hello, great contribution; would you update the PR with new variables to tune the restart behaviour?

[hwo-wd]

Can do, but let me know your thoughts, Guido:

1. do you want to have a basic keycloak_service_restart_on_failure boolean flag (defaults to true) or
2. do you want to have all introduced settings configurable, that is 'StartLimitIntervalSec', 'StartLimitBurst', 'RestartSec' with sane defaults
3. or both, which would lead introduce 4 new (optional) variables

[guidograzioli]

I'd eventually go with option 3, but it's up to you how much of it to include in this PR (whether it is only 1, or 2, or 3, thanks!)

[hwo-wd]

As suggested, I went for 3., but the CI build is complaining on files I didn't change (thus I guess it has been doing so for a while already) - could you please check and let me know your thought? Thanks!

[guidograzioli]

Don't mind the linting issues, ansible-lint has been updated with new rules and they eventually break the CI. I'll merge this now, and update the linter settings in another commit.
Thank you very much for the contribution