puppet: Create memcached PID file that systemd config file specifies.

The systemd config file installed by the `memcached` package assumes
there will be a PID written to `/run/memcached/memcached.pid`.  Since we
override `memcached.conf`, we have omitted the line that writes out the
PID to this file.

Systemd is smart enough to not _need_ the PID file to start up the
service correctly, but match the configuration.  We create the
directory since the package does not do so.  It is created as
`/run/memcached` and not `/var/run/memcached` because `/var/run` is a
symlink to `/run`.

puppet/zulip/manifests/profile/memcached.pp

@@ -99,9 +99,16 @@ Environment=SASL_CONF_PATH=/etc/sasl2
     mode    => '0644',
     content => template('zulip/memcached.conf.template.erb'),
   }
+  file { '/run/memcached':
+    ensure  => 'directory',
+    owner   => 'memcache',
+    group   => 'memcache',
+    mode    => '0755',
+    require => Package[$memcached_packages],
+  }
   service { 'memcached':
     ensure    => running,
     subscribe => File['/etc/memcached.conf'],
-    require   => Class['zulip::systemd_daemon_reload'];
+    require   => [File['/run/memcached'], Class['zulip::systemd_daemon_reload']],
   }
 }

puppet/zulip/templates/memcached.conf.template.erb

@@ -46,5 +46,8 @@ logfile /var/log/memcached.log
 # Maximize core file limit
 # -r
 
+# Use a pidfile
+-P /var/run/memcached/memcached.pid
+
 # Enable SASL authentication
 -S