Add Salt Fiber Box device tracker

[bjornorri]

Description:

Implements a device_tracker platform for the Salt Fiber Box. Salt is a Swiss internet provider.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#11811

Example entry for configuration.yaml (if applicable):

device_tracker:
  - platform: salt
    password: YOUR_PASSWORD

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[elupus]

I'm not at all up to date on device trackers, so just consider this some minor review.

[MartinHjelmare]

Nice!