CDH-13430 - Support RHEL in Whirr CM installation.

Don't rely on the VERSION from the redhat-release providing package -
that returns something like "6Server" on RHEL, which doesn't fit with
anything else we do. Instead, pull the release from lsb_release -sr
and grab just the digits before the period, if there is one.
Merge pull request #38 from abayer/cdh-13430

CDH-13430 - Support RHEL in Whirr CM installation.
Merge branch 'master' into graham-patch-CDH-12820