perf env: Add helper to lazily compute the os_release

perf env: Add helper to lazily compute the os_release

In live mode the os_release isn't being initialized, make a lazy
initialization helper that assumes when the os_release isn't
initialized this is live mode.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Honglei Wang <jameshongleiwang@126.com>
Cc: Jan Polensky <japo@linux.ibm.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>