Added environment variable for base container

In order to be able to use a different base image i have added another
environment variable where everybody can switch the used base container
for the applications directly through the environment variable
DOKKU_IMAGE or with the help of the dokkurc configuration.
Added openSUSE support

I have done the first steps to get dokku running for other distros like
openSUSE. I have simply replaced the important places within the core
plugins with a check for DOKKU_DISTRO that gets exported by dokku
itself. It just replaces the apt-get and service start/restart/reload
functionality with the openSUSE counterparts.
Merge pull request #751 from webhippie/feature/improvements

Partial openSUSE support