puppet: ensure defining $puppet_version does upgrade

As all our precise nodes are on 2.7 and will likely transition to puppet
3, so we need to be sure we remove the pinning files we had for 2.7.

Change-Id: Id7abade4abee72c19afbb42e0b08e480d04f05aa
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>