Invalid service up checking during initial diff (prometheus, alertamanger)

[cmehat]

(same as #609, but for alertmanager and prometheus)

During the first run of the role, it checks if the service has already been installed and fails if not, blocking it's initial installation.

TASK [prometheus.prometheus.alertmanager : Ensure alertmanager is enabled on boot] ***
Tuesday 12 August 2025  18:43:34 +0000 (0:00:07.676)       0:01:31.032 ******** 
Tuesday 12 August 2025  18:43:34 +0000 (0:00:07.676)       0:01:31.030 ******** 
fatal: [monstag]: FAILED! => changed=false 
  msg: 'Could not find the requested service alertmanager: host'

TASK [prometheus.prometheus.prometheus : Ensure prometheus service is started and enabled] ***
Tuesday 12 August 2025  18:44:32 +0000 (0:00:02.277)       0:02:35.274 ******** 
Tuesday 12 August 2025  18:44:32 +0000 (0:00:02.277)       0:02:35.272 ******** 
fatal: [monstag]: FAILED! => changed=false 
  msg: 'Could not find the requested service prometheus: host'