add env variable to disable kernel version warning

Allow the user to set DOCKER_NOWARN_KERNEL_VERSION=1 to disable the
warning for RHEL 6.5 and other distributions that don't exhibit the
panics described in https://github.com/dotcloud/docker/issues/407.
Merge pull request #2993 from jpoimboe/rhel6-version-check

remove kernel version warning on rhel 6.5