MB-9834 Move certificate generation out of config transaction.

Certificate generation might take seconds. And if config is changed
during this period of time, the transaction will need to restart and
hence regenerate the certificate again.

Change-Id: Iaf3914a716719daad3fecac12e03be09d1367954
Reviewed-on: http://review.couchbase.org/31689
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>