* Change filter_by() base argument to a key in lookup_dict rather
than a dictionary. This allows more compact usage and more
readable use of base values/defaults in the lookup_dict.
* Change/improve function-local documentation for lookup_dict().
* Add specific use-case of "base" argument for lookup_dict() in the
formulas documentation.
* Expand filter_by() unit testing to validate use of base argument.
* Add unit test for salt.utils.dictupdate.update() to provide
complete depth-of-testing of filter_by() which uses
dictupdate.update().
