Create folder when generating Omaha ID file

If the folder does not already exist, create it when generating Omaha ID
file.

BUG=chrome-os-partneri:9507
TEST=Build install shim and successfully install.

Change-Id: I3143369a5ae5043831529db429a92cf2561f8f55
Reviewed-on: https://gerrit.chromium.org/gerrit/21946
Reviewed-by: Rong Chang <rongchang@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>