Coverage measurement gate job

Code coverage in Ceilometer usually reaches an acceptable level, but it could be improved. There is a coverage module in tox, which can be used for generating coverage measurements. It checks the lines in the code, which are covered with tests and it provides the overall percentage for the whole code base. The goal of the coverage gate job would be to check that the code coverage is not decreased by a newly proposed patch.

In general this job will help to boost the quality of the patches by detecting those without proper test coverage and emphasizing the importance of writing tests. It will also help in supporting safer code refactors, for example when the code base of an existing functionality is rationalized (the internal logic of a feature is changed, but the functionality remains the same) or code duplication is removed.

In the test phase the gate job would be non-voting.