Ensure webhook run folder exists on redhat systemd environments

On RHEL7, the /run directory is now used as a temporary file storage system
(tmpfs), meaning that files there are lost after reboots.

This parameter ensures that the folder is properly created when the
service starts.

templates/webhook.redhat.service.erb

@@ -5,6 +5,7 @@ After=syslog.target network.target
 [Service]
 Type=forking
 EnvironmentFile=-/etc/sysconfig/webhook
+RuntimeDirectory=webhook
 User=<%= @user %>
 PIDFile=/var/run/webhook/webhook.pid
 TimeoutStartSec=90