Allow zuul & nodepool to be installed on one VM

When attempting to install zuul & nodepool on a single machine
 (typical for third party environments) you get the following error:

Duplicate declaration: Package[python-lxml] is already declared in file
/root/config/modules/zuul/manifests/init.pp at line 64;
 cannot redeclare at /root/config/modules/nodepool/manifests/init.pp:39 on node ci.novalocal

Update the puppet scripts so that python-lxml do not conflict with each other.

Change-Id: I63453e46702d37fa845504f31fa006b0b9fc5330

modules/nodepool/manifests/init.pp

@@ -30,12 +30,12 @@ class nodepool (
   $environment = {},
 ) {
 
-  $packages = [
-    'python-lxml', # needed by python-keystoneclient, has system bindings
-  ]
-
-  package { $packages:
-    ensure => present,
+  # needed by python-keystoneclient, has system bindings
+  # Zuul and Nodepool both need it, so make it conditional
+  if ! defined(Package['python-lxml']) {
+    package { 'python-lxml':
+      ensure => present,
+    }
   }
 
   class { 'mysql::server':

modules/zuul/manifests/init.pp

@@ -54,7 +54,6 @@ class zuul (
   $packages = [
     'gcc',  # yappi requires this to build
     'python-lockfile',
-    'python-lxml', # needed by python-keystoneclient, has system bindings
     'python-paste',
     'python-webob',
   ]
@@ -69,6 +68,14 @@ class zuul (
     require  => Class['pip'],
   }
 
+  # needed by python-keystoneclient, has system bindings
+  # Zuul and Nodepool both need it, so make it conditional
+  if ! defined(Package['python-lxml']) {
+    package { 'python-lxml':
+      ensure => present,
+    }
+  }
+
   # A lot of things need yaml, be conservative requiring this package to avoid
   # conflicts with other modules.
   if ! defined(Package['python-yaml']) {