Add support to download the latest stable bootstrap script.

If we're not installing under Windows, provide the same features salt-cloud had at install time before the merge.
Update to latest stable bootstrap script, v1.5.9
Fix some missing variables and imports.
One less file in salt_cloud
Make PyLint a bit happier.
Merge pull request #8527 from s0undt3ch/hotfix/merge-salt-cloud-setup

Migrate salt-cloud setup features to salt