Use system_mail_from when using SMTP system configuration

tmpl/alertmanager.yml

@@ -10,6 +10,7 @@ global:
   smtp_smarthost: '%%exim_relay_smtp:465'
   smtp_require_tls: true
   %end if
+  smtp_from: '%%system_mail_from'
 %else
   smtp_smarthost: '%%alSMTPHost:%%alSMTPPort'
   smtp_from: '%%alFrom'
@@ -31,8 +32,8 @@ global:
   #hipchat_api_url: 'https://hipchat.foobar.org/'
 
 # The directory from which notification templates are read.
-templates: 
-- '/etc/alertmanager/template/*.tmpl'
+templates:
+  - '/etc/alertmanager/template/*.tmpl'
 
 # The root route on which each incoming alert enters.
 route:
@@ -54,12 +55,12 @@ route:
 
   # If an alert has successfully been sent, wait 'repeat_interval' to
   # resend them.
-  repeat_interval: 3h 
+  repeat_interval: 3h
 
   # A default receiver
   receiver: %%alDefaultReceiver
 
-  # All the above attributes are inherited by all child routes and can 
+  # All the above attributes are inherited by all child routes and can
   # overwritten on each.
 
   # The child route trees.
@@ -119,7 +120,7 @@ route:
 
 # Inhibition rules allow to mute a set of alerts given that another alert is
 # firing.
-# We use this to mute any warning-level notifications if the same alert is 
+# We use this to mute any warning-level notifications if the same alert is
 # already critical.
 inhibit_rules:
 - source_match: