Fix notebook version to 6.0.3

A regression was introduced in notebook 6.1.0 where it is no
longer possible to start a terminal by simply going to the url
"/terminals/1". A fix is currently underway, but until a new
version of notebook is released, we will settle for 6.0.3.

Related issue:
jupyter/notebook#5790

PR fixing this issue:
jupyter/notebook#5813

data/common.yaml

@@ -1,5 +1,6 @@
 # jupyterhub/data/common.yaml
 ---
+jupyterhub::notebook::version: 6.0.3
 jupyterhub::jupyterhub::version: 1.1.0
 jupyterhub::batchspawner::url: https://github.com/jupyterhub/batchspawner/archive/v1.0.0.zip
 jupyterhub::slurmformspawner::version: 2.2.0

manifests/node.pp

@@ -23,8 +23,11 @@
 }
 
 class jupyterhub::node::install (Stdlib::Absolutepath $prefix) {
+
+  $notebook_version = lookup('jupyterhub::notebook::version')
+
   exec { 'pip_notebook':
-    command => "${prefix}/bin/pip install --no-cache-dir notebook",
+    command => "${prefix}/bin/pip install --no-cache-dir notebook==${notebook_version}",
     creates => "${prefix}/lib/python3.6/site-packages/notebook/",
     require => Exec['jupyterhub_venv']
   }