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
Merge "Allow zuul & nodepool to be installed on one VM"