nginx: adding ssl config to the module

The class nginx::ssl will be used to configure ssl on the hosts in an
uniform manner; it also allows to use a couple of switches that should
allow us to uniform the ssl config everywhere.

Change-Id: Ia97b46401cc325b2e1fe83dde479f42518ebb21f
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>