Integration of coverage jobs with builds

Today I discovered an issue with the keystone test suite [1] whereby coverage wouldn't run correctly. A patch [2] was quickly added, but technically, there is no automated process in place to prevent the same sort of regression in the future. Jenkins automatically ran the tests, but not coverage.

After a discussion with fungi and others in #openstack-dev and -infra, there are apparently some problem with a tight integration of coverage into the build. Regardless, it's probably still worth investigating.

[1] - https://bugs.launchpad.net/keystone/+bug/1245539
[2] - https://review.openstack.org/#/c/54140/