The existing GetOSVersion has a lot of unused code which is wrong in
several ways
 - the only path tested in upstream CI is with lsb_release, because
 it's pre-installed on all nodes
 - the /etc/redhat-release checking probably still works, but is
 unnecessary
 - If using lsb_release, os_UPDATE has never actually been set.
 - the /etc/SuSE-release branch checking is broken if the lsb package
 is actually installed. lsb checking does not set os_UPDATE but yet
 the SuSE DISTRO setting relies on this to set a patch level (and so
 does some of the rpm tags). SuSE 11 is up to update 3, but the rpm
 matching is stuck hard-coded to update 2. I'm guessing
 installation is actually broken there.
 - the debian checking branch is broken. The VERSION tags have been
 removed and were not supposed to be relied on anyway (see notes in
 [1])
This simplifies things:
 - remove OSX checking (moved here after discussions in
 I31d0fdd30928ecc8d959a95838b1d3affd28ac6f)
 - only use the output of lsb_release.
 - A small best-effort check to pre-install lsb packages if not
 detected (that avoids chicken-egg-problem of package-install
 wrappers relying on os_* flags).
 - The unset os_UPDATE is removed. It's only previous use was for
 setting separate suse versions in the DISTRO element for matching
 during package installs (since removed)
 - DISTRO setting is modified to use the parts of os_RELEASE it wants.
 Per-above, this is the correct place to parse out specifics.
 - Call out the is_* functions, which are a better way to detect
 platforms
 - Export the variables as read-only, since they shouldn't be reset
[1] http://sources.debian.net/src/base-files/7.5/debian/changelog/
Change-Id: I46a2c36d95327087085df07cb797eb91249a893c