Incorporate most review suggestions:
- describe parameters better for directory listing
- remove un-needed else conditionals
- correct misnamed class entries
- rename osd info class
- but leave class methods, thinking being that validation attached to class
  makes logic trivially simple to understand... as bare functions they have
  no context and are not obvious as to what they are trying to do.
- after investigation realize that unresolved links do not generate errors
  therefore remove exceptions in link resolution and dir listing. Note that
  this corresponds to similar code in ceph-disk. It would seem that exceptions
  will be raised by serious problems like filesystem errors or missing dirs
  and these should cause a higher level error to be raised.
- change var != None to just var as recommended
- removed excessive use of () around objects as suggested.