As per discussion in code review.

This is just a hunch. The test shouldn't fail, and it doesn't fail
locally. It seems this might fix it (and point to the cause).

- Modify defaults in the schema so that correct values are loaded (add
  notes to description where applicable).
- When the default in schema was inconcsistent with the default loaded
  in `__init__.py`, use the latter.
- Replace `kwargs.get(foo, default)` with `self.foo` if default is set
  correctly in the schema.
- Remove redundant assignments and variables (there are still more
  left).
- The defalt for `citation_cache_lock_dir` should be `citations/locks`.
- In a few cases where defaults are NOT loaded in `__init__.py`, and the
  values are clearly meant to be suggested values, move those defaults
  into description.

The function was redundant: it checked whether its argument was an
absolute path, and if not, called os.path.join. This is exactly what
os.path.join already does (python 3.x and 2.7)

The only benefit of having a separate function is the ability to add
additinal processing to such calls - so when we need that, we can add
it. Meanwhile, it removes unnecessary function calls + makes the code
slightly less complex.

The function is not called from outside this module (not to be confused
with the resolve_path method of the mixin class), but it's duplicated in
webapps/reports/config.py and webapps/tool_shed/config.py (which are not
addressed in this commit).