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.
@crosbymichael There's only one valid case where we want to allow this and that's for the 2.6.32 kernel on RHEL6/CentOS6/Scientific Linux 6.
Allowing that env var to be use just allows users to set the env var and ignore the warning (now turned into error).

We've already had some bug reports where we were being told they were ignoring the warning on purpose.