Auto download the bootstrap-script at package build time.

Whenever the salt-cloud package is build, we now download the defined version of the salt-bootstrap.
A choice is also given to the user to provide a different bootstrap script version, one only needs to set `BOOTSTRAP_SCRIPT_VERSION` to an existing tag.
We want the RAW version of the URL. Properly handle HTTP errors.
Merge pull request #470 from s0undt3ch/features/auto-download-bootstrap-script

Features/auto download bootstrap script