Enabled source code coverage for contrib directory

So we can check the source code coverage status of contrib
directory. It's obvious to adjust omit and source properties in
.coveragerc. The tricky code in .testr.conf is caused by the behavior
of setuptools.

When running coverage, setuptools set environment variable PYTHON to
'coverage run --source package --parallel-mode' and call testr to run
the tests. The value specified via command line option --source will
overwrite the source value in .coveragerc. So contrib directory is
excluded from coverage run.

The change in .testr.conf removed the '--source heat' specified by
setuptools. So the source value specified in .coveragerc will be used
by coverage.

Change-Id: I92f6b058067ace3b3370a3670856ab3dc073b52f
Merge "Enabled source code coverage for contrib directory"