puppet: re-enter $cluster at the node level

When using role-based hiera lookups, we need to do all hiera lookups at
the node scope, as it's where the roles are evaluated. For now the
global variable $cluster will need to be declared here, while we think
of a better way.

Change-Id: I4cfbd01488bdd37395e366ae7cc77bc1b1e92c36
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>