Instruct coverage instance to cover requested packages only.

When `--cover-package=blah` is specified at the command line, the package is not passed to the coverage instance.    So even though the coverage report will only include the `blah` package, the `.coverage` file written will still contain coverage information for packages other than `blah`.  This is important if you are using `coveralls` and do not have a `.coveragerc` file in the directory where `nosetests` is run.