puppet: hiera backend for the WMF

The hiera nuyaml backend will allow us to use hiera with ease and will
make it both easier to maintain (allowing for more atomic files) and
simpler to use in our setup where a lot of variables have a second order
dependency on other data.

A puppet class and conf files for both production and labs are included
in the patch, but hiera config will only be deployed to labs for now.

In our configurations, all hiera data will be under the hiera
subdirectory of /etc/puppet, and all private data (like passwords and
such) will be stored in /etc/puppet/private instead.

Change-Id: I24b1db4d0ef8394115e4830871e71e2d227ec374
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>