Core improvements and 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.