Collect per-test coverage for both riak_test and eunit tests for giddyup

To enable us to be able to see the *combined* coverage of our unit and
integration tests, modify riak_test and the smoke_test runner to capture
coverage data per-test and post it as a giddyup artifact.

To maintain the current riak_test behaviour where the *combined*
coverage is reported on at the end of a run, each test writes its own
.coverdata file, cover is reset and then once all tests are run, the
coverdata files are all loaded and the total coverage is reported.
Address review concerns

* Change group leader for cover_server while generating reports, so the
  'includes data from imported files' message can be suppressed.
* Append a phash of the test metadata to the coverdata filename to keep
  them unique.
* Removed unused maybe_stop function.
Silence is golden
Merge pull request #554 from basho/adt-upload-coverage

Collect per-test coverage for both riak_test and eunit tests for giddyup