fix crash bug in linux_sysctl.assign() on Virtuozzo containers
fix crash bug in linux_sysctl.assign() on Virtuozzo containers

On systems like Virtuozzo containers, "sysctl -w" is not allowed,
ever, and will fail with ENOPERM errors even if one is root.  Since
assign() doesn't check exit status or the returned string, this
leads to a nasty stack trace.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt