DC/OS Enterprise Support

[zonybob]

I had a short-term requirement to get my mesos-rxjava-based framework onto DC/OS Enterprise with strict security mode. I would like to submit a PR once tests/documentation are in better shape, and I am simply creating an issue for tracking purposes.

The gist of my change is...

• check if mesosUri is "https". If so, add a "trustAll" SSLEngineFactory to the HTTPClient
• provide a supplier/callback for adding arbitrary headers to the HTTP POST call

I'm not sure how active this project is currently, but I welcome any feedback and suggestions necessary to make the changes acceptable.

PR to follow this week (hopefully)...