Add `plugins/mitogen` to `.gitignore` (#6774)

If the `mitogen.yml` playbook is run, it installs Mitogen in this path, causing Git to believe there to 500+ changes. This simply excludes that external module from git

.gitignore

@@ -15,6 +15,7 @@ contrib/terraform/aws/credentials.tfvars
 **/*.sw[pon]
 *~
 vagrant/
+plugins/mitogen
 
 # Ansible inventory
 inventory/*