Make temporary file location a variable.

[dglinder]

This removes the need for "/opt/" to be writable by a users and permits the script to be run by non-root users.

I took the liberty of updating the comment lines so future work to re-activate them should work as well.

[clemenko]

Between the two PRs you have added 3 features.

1. opt --> /var/tmp
2. docker login
3. hauler binary location

I am not sure these are all needed. The theory of this script is to create all the hauler tar for getting all the bits to create a rancher cluster air gapped.

1. Actually I think I am find with the temp location. If you want to create a PR just for that, that is cool.
2. why do you need a docker login? the script works without that.
3. Why the hauler location? the script will download hauler.