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
diff --git a/stack.sh b/stack.sh
index 94b90d1..5f12a80 100755
--- a/stack.sh
+++ b/stack.sh
@@ -34,6 +34,9 @@
 # Make sure umask is sane
 umask 022
 
+# Not all distros have sbin in PATH for regular users.
+PATH=$PATH:/usr/local/sbin:/usr/sbin:/sbin
+
 # Keep track of the devstack directory
 TOP_DIR=$(cd $(dirname "$0") && pwd)