pin puppetlabs-ntp to 4.2.x branch

Context:
https://bugs.launchpad.net/tripleo/+bug/1633713

Change-Id: I82ec1f7a0a1ee69c81f14e9c2e78df616f345576
diff --git a/rdo.yml b/rdo.yml
index b54e2ea..2476cfc 100644
--- a/rdo.yml
+++ b/rdo.yml
@@ -229,6 +229,9 @@
 - project: puppet-ntp
   conf: rpmfactory-puppet
   upstream: git://github.com/puppetlabs/puppetlabs-ntp
+  tags:
+    ocata:
+      source-branch: 4.2.x
 - project: puppet-rabbitmq
   conf: rpmfactory-puppet
   upstream: git://github.com/puppetlabs/puppetlabs-rabbitmq