puppet: switch masters to puppet 3

As the update of the puppet master to version 3 has went well in labs,
next step would be to apply this change in production.

During the upgrade, a small (as in some seconds, hopefully) outage of
the puppet service will likely occur.

This will be merged only the necessary consensus has been reached.

Change-Id: Ib55872afd3b89fde4f70b95f803352849cd10b09
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>