Fix #13020 (salt.modules.system.join_domain).
- Clean up join_domain() which failed until now due to a wrong .format()
  string.
- Replace invalid variable names.
- Use more descriptive variable names.
- Improve documentation.
- Add workaround to fix 'wmic's violation of RFC2253 2.1
- Resolve possible failures/return codes
Merge pull request #13023 from eliasp/system.join_domain-tuple_index

Fix #13020 (salt.modules.system.join_domain).