monitoring: use config_template module for config

monitoring: use config_template module for config

The alertmanager, grafana and prometheus configuration file are
generated with the template module which doesn't allow for using
config overrides.
Instead we could use the config_template plugin action and add a
new variable for overrides (one for each component).

With this patch, one should be able to add configuration to
prometheus with the following:

---
alertmanager_conf_overrides:
  global:
    smtp_smarthost: 'localhost:25'
...

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1902999
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>