Remove automagic file finder

I think we've agreed to just let bashate take a list of files, and
leave the discovery up to the calling test-harness.

This removes the auto-discover component, and cleans up the
error-handling slightly.  It will print the usage if you leave off a
list of files to scan, and will give a sane error message if a file is
not found.

Change-Id: If9ad0855dfbd10f2a55c98b139f99167dc85a2aa
Merge "Remove automagic file finder"