Partial openSUSE support

I have done some improvements as you can see on the separate commits like ~~keeping the environment for sudo, replaced hard coded plugin paths and~~ an environment variable for the base container.

On top of that i have added first pieces to be able to run dokku on a different platform than just only Ubuntu. I'm interested more in openSUSE as i'm an employee of SUSE :P

I hope we can merge that quite fast as i want to build packages on top of that as i already mentioned in #744.
Ah, forgot to mention that. This setting is not set and not available within any comment on plain SUSE configuration of nginx. As the conf.d folder gets read by both distros that should be a secure solution for both.