endpoint_responding? = true for Net::HTTPForbidden

this allows for the restart to finish, even when the api endpoint
can't be reached because of authentication.

Another way would be to actually implement proper authentication
inside the restart script, but I feel like this is overkill, as the
`HTTPForbidden` already indicates something is listening on that
endpoint and rejecting the request.

It's basically the same as `HTTPSuccess` where we accept that
something is listening, without knowing that it's actually Jenkins.
Merge branch 'pr/26'