pin npm markdownlint version to 0.18.0 to run with nodejs >=8

* on ubuntu 18.04 the latest nodejs version from the default debian
  package repository is nodejs 8, so we need to pin the markdownlint to
  a version which does not require a later release of nodejs

Signed-off-by: Jan Klare <jan.klare@bisdn.de>
Change-Id: I302b22b2f83f347296097bafbc5392c77b8b138a
diff --git a/packer/provision/onf-playbook.yaml b/packer/provision/onf-playbook.yaml
index 4658043..eb211df 100644
--- a/packer/provision/onf-playbook.yaml
+++ b/packer/provision/onf-playbook.yaml
@@ -168,6 +168,7 @@
     - name: Install markdownlint npm package with version specifiers
       npm:
         name: markdownlint
+        version: '0.18.0'
         global: true
 
     - name: Install typings npm package with version specifiers