Add an extra parameter for test directory in debugger script

The `python-*client`'s usually give back the full package name
i.e. python-keystoneclient, and the script assumes thats where
the tests are, but most clients have their tests under just
*client, or in this case keystoneclient.
Oslo projects have a similar issue.
Change the debug script to accept an optional argument that can
be used to override the guesstimated value.

Change-Id: If0a25ae7ab9f854f87271a377dd228e2ebfb107f
Merge "Add an extra parameter for test directory in debugger script"