Add colorization and log_level support to salt provisioner.

This commit adds two new options to the salt provisioner:

- `colorize`: controls color output. Defaults to no-color (present behavior).
- `log_level`: controls log level. Defaults to `debug` (present behavior).

Docs included!
Merge pull request #3603 from eykd/feature/colorize-salt-output

provisioner/salt: Add colorization and log_level support to salt provisioner.