Adding validator and parser for default_groups and tags config

[ebuckley]

Context

Ckan NZ is running a fork of ckanext-spatial which includes a few extra validation steps and a few modifications that make source config closer to the base ckan harvester behaviour.

Changes

1. add the same validation as base CKANHarvester for the source config object
2. adds support for default groups in the spatial harvester
3. Modifies expected harvest source configuration for tags to match the behavior of CKANHarvester

[ebuckley]

Happy to update tests to support this change if there is some desire to have it contributed back in to core :)

[amercader]

@ebuckley this looks great, it would be great if you could update the tests (and test this new behaviour) thanks