Improve filter_by() base argument:

  * 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().