Add UT for `repository_service_tuf_api/__init__.py`

[kairoaraujo]

Add UT to the parts that are uncovered by our current unit tests.

There is a complication here because the settings and secret_settings are initiated directly and loaded when the app.py is called.

We might be required to move it to functions as we did on RSTUF Worker, and it will give us more flexibility to mock and use pretend.

Originally posted by @kairoaraujo in #329 (comment)

[MVrachev]

For future reference, the tests that coverage was complaining about are:

• repository-service-tuf-api/repository_service_tuf_api/__init__.py

  Line 114 in 7df983c

  logging.error(str(err))

• https://github.com/kairoaraujo/repository-service-tuf-api/blob/7df983cee26972fabf0d3d0730c333fca479a867/repository_service_tuf_api/__init__.py#L124

• https://github.com/kairoaraujo/repository-service-tuf-api/blob/7df983cee26972fabf0d3d0730c333fca479a867/repository_service_tuf_api/__init__.py#L166

You should be able to see those warnings here: https://github.com/repository-service-tuf/repository-service-tuf-api/pull/329/files