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.
Why aren't we using $DOKKU_IMAGE here?