add coverage config file to control module coverage report

Ceilometer copies oslo common code into its code stream. When run
unit tests with coverage option, the overall test code coverage
percentage is low due to the enclosure of the common libraries.
Adding the coverage config file will exclude these modules in
the code coverage report so that developers can easily see if
new test cases has increased or decreased the code test coverage.

to see the code coverage report, following the following steps:

1. In ceilometer root directory, execute the following command
   ./run-tests --coverage
2. Once the test run completed successfully, open up the follwoing
   file in your browser and scroll down to the bottom to see the
   total number. (this patch makes that number to be 90%)
   <ceilometer_root_dir>/cover/index.html

To keep up with this effort, whenever you add new code, please
run the test code with coverage option and make sure your new
code and test won't decrease the total percentage. If it did,
that means you may have new code un-tested :-)

Change-Id: I277c243b6aadb0e8524915452326ed875e14ce9c