ckan-ng-harvester can be toggled on and off for an individual harvest source

[adborden]

As a Data.gov operator, I want ckan-ng-harvester running side-by-side with the legacy harvesters so I can switch between the legacy harvesters or ckan-ng-harvester so that I can incrementally migrate to ckan-ng-harvester one source at a time.

Details

• Work started here: https://github.com/GSA/ckanext-harvest/pull/10/files
• set up new command to toggle between old and new harvesters
• write tests (as always)

Acceptance criteria

• [ ] Legacy harvesters and ckan-ng-harvest are running on Data.gov production without conflicting with each other or causing issues.
• [ ] I can switch between legacy harvesters or ckan-ng-harvester on a single harvest source with a single command.
• [ ] Switching between legacy harvesters and ckan-ng-harvester is automatic, requiring no manual steps or clearing existing state.
• [ ] Adding a new harvest source defaults to the legacy harvesters.
• [ ] When I add a new harvest source, it defaults to using the legacy harvesters and not ckan-ng-harvester.

[avdata99]

Change proposed: Add a setting value in ckanext-harvest to define wich app is going to run this harvest process. Defaults to CKAN (local instance).

PR at ckanext-harvest: https://github.com/GSA/ckanext-harvest/pull/10/files

If this seems OK for all my next step is to use this config value in NG harvester to only harvest this marked sources. @akariv @adborden