This branch of a2server only supports jessie and above, either Raspbian
or Debian. While I was at it, I simplified the tests for what is and is
not a supported OS. If there are bad kernels or firmwares we need to
work around those should be fixes. This also means giving the user an
option to interrupt the installation if they are trying to install on a
Debian(ish) system that is too old or (more likely) too new.
At least ivan.sh is now using a2cloud's system_ident script.
