Run service as keycloak_service_user

[schmaxit]

The actual implementation always runs the service as root user.
This PR uses the existing variables keycloak_service_user and keycloak_service_group to set the user and group of the startup script and so of the actual java process.
I have also changed the default keycloak_service_pidfile value allowing to set the correct permissions on the folder containing the pidfile

[guidograzioli]

Hello thanks for the PR; this is a much needed change, but it will create many side-effect on existing deployments (systemd not finding the pidfile for the running instance, files owned by root around not readable by the process, and so on). So for backwards compatibility, and to lower the PR level to major_change from breaking_change, can you please:

1. add a keycloak_service_runas (bool, default false) flag and protect your feature behind it; by default it retains the current behaviour of running as root for backwards compatibility
2. add a conditional task when keycloak_service_runas is true, which recursively re-owns to keycloak_service_user everything under keycloak.home

The above for the keycloak role only, while for keycloak_quarkus it is good as it is now

[guidograzioli]

LGTM; thank you very much for this contribution