Deal with sshd being absent

This plugin is no longer part of a default installation and not needed
in our CI.

files/puppet_helper.groovy

@@ -241,7 +241,8 @@ class Actions {
     user.addProperty(pw_param)
 
     if ( public_keys != "" ) {
-      def keys_param = new org.jenkinsci.main.modules.cli.auth.ssh.UserPropertyImpl(public_keys)
+      util.requirePlugin('sshd')
+      def keys_param = this.class.classLoader.loadClass('org.jenkinsci.main.modules.cli.auth.ssh$UserPropertyImpl').newInstance(public_keys)
       user.addProperty(keys_param)
     }
 
@@ -294,10 +295,11 @@ class Actions {
 
     def public_keys = conf['public_keys']
     if (public_keys) {
+      util.requirePlugin('sshd')
       assert public_keys instanceof List
       // convert list of keys into a single string
       def keys = public_keys.join("\n")
-      def keys_param = new org.jenkinsci.main.modules.cli.auth.ssh.UserPropertyImpl(keys)
+      def keys_param = this.class.classLoader.loadClass('org.jenkinsci.main.modules.cli.auth.ssh$UserPropertyImpl').newInstance(keys)
       user.addProperty(keys_param)
     }