MB-11286: don't crash when stopping xdcr ssl proxy

child_erlang facility expects a no-args stop function in bootstrap
module. And it was not provided for ns_ssl_proxy

Change-Id: I4de5aed24e8ea42d519c68655e3c26749391dcd8
Reviewed-on: http://review.couchbase.org/37885
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>