docker: buildslave: make TLS working

docker: buildslave: make TLS working

It seems like twisted actually needs SSL in the connection string,
otherwise it will get stuck in the following endless loop:

 [buildbot_worker.pb.BotFactory#info] Starting factory <buildbot_worker.pb.BotFactory object at 0x7ff40f19e080>
 [twisted.application.internet.ClientService#info] Scheduling retry 1 to connect <twisted.internet.endpoints._WrapperEndpoint object at 0x7ff410352978> in 2.1271975799063436 seconds.

Signed-off-by: Petr Štetiar <[email protected]>