Parameterize Prometheus alertmanager SMTP params

roles/prometheus/defaults/main.yml

 ---
-alert_email: "root"
 
+# Destination (To:) of alert emails.
+alert_email: "root@{{ domain }}"
+
+# Alertmanager SMTP parameters.
+alertmanager_smtp_from: "prometheus@{{ domain }}"
+alertmanager_smtp_smarthost: "localhost:25"
+alertmanager_smtp_require_tls: false
+alertmanager_smtp_hello: "localhost"
+alertmanager_smtp_auth_username: ""
+alertmanager_smtp_auth_password: ""

roles/prometheus/templates/alertmanager.yml.j2

 global:
-  smtp_from: 'prometheus@{{ domain }}'
-  smtp_smarthost: 'mail-frontend.{{ domain }}:10025'
-  smtp_require_tls: false
+  smtp_from: '{{ alertmanager_smtp_from }}'
+  smtp_smarthost: '{{ alertmanager_smtp_smarthost }}'
+  smtp_require_tls: {{ alertmanager_smtp_require_tls }}
+  smtp_auth_username: '{{ alertmanager_smtp_auth_username }}'
+  smtp_auth_password: '{{ alertmanager_smtp_auth_password }}'
+  smtp_hello: '{{ alertmanager_smtp_hello }}'
 
 templates:
   - '/etc/prometheus/alertmanager_templates/*.tmpl'