deploy the "defaults" file correctly so that changes take effect

Without this change, when the install_method is set to `package`, the
various `prometheus::server` settings will never do anything because
we stop touching the systemd config file.

By default, the Debian systemd.service file has this entry:

EnvironmentFile=/etc/default/prometheus
ExecStart=/usr/bin/prometheus $ARGS

... which makes it use the ARGS define in the "default" config
file. We harmonize our systemd config and init.d files to sync up with
that file instead of rewriting the startup file.

Closes: #323.

manifests/config.pp

@@ -61,6 +61,15 @@
       $systemd_notify = Exec['prometheus-systemd-reload']
     }
 
+    if $prometheus::env_file_path != undef {
+      file { "${prometheus::env_file_path}/prometheus":
+        mode    => '0644',
+        owner   => 'root',
+        group   => '0', # Darwin uses wheel
+        content => "ARGS='${join($daemon_flags, ' ')}'\n",
+      }
+    }
+
     case $prometheus::server::init_style {
       'upstart' : {
         file { '/etc/init/prometheus.conf':

templates/prometheus.debian.erb

@@ -18,8 +18,12 @@ DESC="Prometheus monitoring framework"
 NAME=prometheus
 DAEMON=<%= scope.lookupvar('prometheus::server::bin_dir') %>/$NAME
 PIDFILE=/var/run/$NAME/$NAME.pid
+<% if scope.lookupvar('prometheus::server::env_file_path') %>
+DAEMON_ARGS=''
+<% else %>
 DAEMON_ARGS="<%= @daemon_flags.join("\n             ") %>
              <%= scope.lookupvar('prometheus::server::extra_options') %>"
+<% end %>
 USER=<%= scope.lookupvar('prometheus::server::user') %>
 SCRIPTNAME=/etc/init.d/$NAME
 

templates/prometheus.systemd.erb

@@ -7,8 +7,13 @@ After=basic.target network.target
 [Service]
 User=<%= scope.lookupvar('prometheus::server::user') %>
 Group=<%= scope.lookupvar('prometheus::server::group') %>
+<% if scope.lookupvar('prometheus::server::env_file_path') %>
+EnvironmentFile=<%= scope.lookupvar('prometheus::server::env_file_path') %>/prometheus
+ExecStart=<%= scope.lookupvar('prometheus::server::bin_dir') %>/prometheus $ARGS
+<% else %>
 ExecStart=<%= scope.lookupvar('prometheus::server::bin_dir') %>/prometheus \
   <%= @daemon_flags.join(" \\\n  ") %>
+<% end %>
 ExecReload=/bin/kill -HUP $MAINPID
 KillMode=process
 Restart=always