Document: Mesos Executor expects all SSL_* environment variables to be set

I was trying to run Docker containers in a fully SSL-ized Mesos cluster but ran into problems because the executor was failing with a "Failed to shutdown socket with fd 10: Transport endpoint is not connected".

My understanding of why this is happening is because the executor was trying to report its status to Mesos slave over HTTPS, but doesnt have the appropriate certs/env setup inside the executor.

(Thanks to mslackbot/joseph for helping me figure this out on #mesos)

It turns out, the executor expects all SSL_* variables to be set inside `CommandInfo.environment` which gets picked up by the executor to successfully reports its status to the slave.

This part of _executor needing all the SSL* variables to be set in its environment__ is missing in the Mesos SSL transitioning guide. I request you to please add this vital information to the doc.