Issue title: Switch to codecov for coverage
 Issue body: We are seeing frequent issue in our CI in coveralls recently.

We may want to consider switch to other service to track the test coverage, such as https://codecov.io/
 Comments: 
 Comment 0: Low priority as coveralls has been OK recently.
 Comment 1: Close it as fixed.
{'source': 'GithubIssues', 'repo_name_with_owner': 'goharbor/harbor', 'issue_id': 479924072, 'repo_stars': 13787.0}