Add omit files for coverage

This commit adds omit files for coverage. In tempest, we can get
coverage of its unit tests. But we don't need the coverage of api tests,
scenario tests and api_schema. Because we don't need to have their unit
tests.

Depends-on: I6a1e7a411a4b27fa0f9bd533276d5cb4d4759976
Change-Id: I91931a68485f03b88872a2f1f860cf8da2f28eb5
1 file changed