Use urandom rather than openssl for password gen

I tried using devstack on a cloud image that didn't have openssl, so
it generated blank passwords and gave an error about openssl missing.
Using urandom should not require more than minimal shell tools.

Change-Id: Ib3091183eb1d85fc8cf7c76e47da24a04ae4791f

stack.sh

@@ -410,7 +410,7 @@ function read_password {
             echo "Invalid chars in password.  Try again:"
         done
         if [ ! $pw ]; then
-            pw=`openssl rand -hex 10`
+            pw=$(cat /dev/urandom | tr -cd 'a-f0-9' | head -c 20)
         fi
         eval "$var=$pw"
         echo "$var=$pw" >> $localrc