suse platform: read /etc/SuSE-release only once.

doing it twice is (slightly) inefficient and complicates the tests
[OHAI-339] distinguish openSUSE from SUSE Linux Enterprise

- platform_family: stays "suse" for both
- platform: SLE, being the more stable platform,
                 gets to keep platform "suse"
            openSUSE is now platform "opensuse"
- platform_version: is "MAJOR.MINOR" even for SLE which officially is
                    versioned as "MAJOR [SP MINOR]"

I considered to change the platform only for *future* openSUSE
versions (12.3 or later) but decided against it: "suse" >= 12 would
then conflict with a future SLE 12 (scheduled for 2014).