labs: preseed changes for Ubuntu 14.04

Ubuntu 14.04 LTS needs an additional localepurge option to cruise
through installation using preseed.

This patch adds the missing option. The Ubuntu 12.04 LTS does not need
the option but ignores it.

Change-Id: I18b9c4e4b19d22fc92c661731e10529e30740c56
Merge "labs: preseed changes for Ubuntu 14.04"