nagios_common: allow use of a template for contacts.

By using a template, we'll guarantee us that we can save the private
data of all the notified people in the private hiera repo only, under
nagios_common::contacts::contacts

We still leave open the option to use a manually-edited file, although
that's suboptimal, to allow for changes to be gradual.

Change-Id: I7ce1715104e64c77afb6b08d1084c5befea1591a
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>