Move elevation to core config and clean up HTTP mocking in tests

[balloob]

Description:
We've had a method to fetch the elevation from Google for a while now. This method was used by both the YR platform and Sun component to fetch the elevation of the location. This meant a request to Google APIs on startup unless a user passed in elevation to the configs of either platform or component.

This PR makes elevation a first class citizen of the config object in core and allow it to be set by configuring it in the [homeassistant] section of your config.

Elevation will now be only automatically fetched when we detect core configuration missing it.

I also moved some of the config related code from bootstrap to config, where it belongs.

While writing tests for util.location, I went to use httpretty as recently introduced to test forecast.io. However after reading up on it I noticed that it does not officially support Python 3. As a move to consolidate our HTTP testing efforts, I decided to use requests-mock and migrate the tests that depend on either httpretty or betamax to requests-mock.

Checklist:

If the code does not interact with devices:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
• Tests have been added to verify that the new code works.