Use os.path.join with cli_dir

Use os.path.join to create final cli path.  This allows the path to be
specified in the same way as the extant "bin_dir" provided to the
whitebox tests; this is useful to a top-level driver like devstack so
it doesn't have to worry about differences with trailing /'s

TODO: create one global config for both?

Change-Id: Ied5a62993fa9a62fd06cc0341888c5cbc62c8a91