MB-10953 Clear ssl PEM cache after saving certificates.

Previously we restarted entire ssl application when certificates
changed to achieve the same. The important difference is that it
didn't happen when certificates was generated during
ns_ssl_services_setup initialization. So when the key is regenerated
after node was ejected from the cluster, ssl application might still
use cached values. Though for reasons I don't understand, it happens
quite rarely.

Change-Id: Ibac30e698a24a2628bd739ea744901ab616d100f
Reviewed-on: http://review.couchbase.org/36724
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>