Refactor API tests

1. Remove method `_get_credentials` from module `tests.foreman.api.test_host_v2`.
2. Add functionally identical method `get_server_credentials` to module
   `robottelo.common.helpers`.
3. Add tests for `get_server_credentials` to `tests.robottelo.test_helpers`.
4. Make module `tests.foreman.api.test_host_v2` use the new method.
5. Make a few pylint-related fixes.
6. Remove class `ApiHostsIdTestCase`, which does nothing useful. Remove largely
   useless assertions from `ApiHostsTestCase.test_get` method.
Add some API tests for "Model" Satellite entity