Ensure sbin is in PATH.

Some distros (Debian) don't have sbin in PATH for non-root users.

Nova (and possibly other services) assumes that it can invoke "sysctl"
without sudo.

Change-Id: Iced21fc1378af309fb49688f9b63f2cd8383e304
Closes-Bug: #1300800
Merge "Ensure sbin is in PATH."