Move cluster definition to the node level.

We happen to use the $cluster variable for a few reasons, mainly for
ganglia but also for other things. I will remove all declarations below
node-level as they don't work well with puppet 3; for now I just added
it at node level for testing purposes.

This should not interfere with any functionality however.

Change-Id: I5bd8cea283e1f196378c8ad956b69b1d3be34065
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>