Make the kernel requirement a hard requirement

This PR changes the kernel version checking code to make sure the used kernel is version 3.8 or newer.
The only exception is kernel 2.6.32 on RHEL6 and its derivatives. The exception is implemented by checking os-release through the pkg/parsers/operatingsystem package.

We've seen a lot of open issues from users who were running kernel 3.2 and were surprised Docker wasn't stable for them. We've also had some reports where the users actually knew about the requirement and decided to ignore it.

This change needs to be implemented so we avoid bug reports from those running kernel 3.2.
What about people still using this env var?