Rename the _oss cluster vars to _os

Per the mailing list discussion, rename _oss to _os, both in cluster parameters
and in the rest of the code.

This is just an s/_oss/_os, with the exception of a small bit of cleanup
around the helper_os function in cmdlib.py.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>