Add an api endpoit for fetching all enabled data providers

Summary:
* Add a new endpoit api/v2/dataproviders
* Restructure data-providers config params

Todo: Drop `data_feeds` table and it's associated model class

Test Plan:
run test ./bin/behat --config application/tests/behat.yml application/tests/features/api.dataproviders.feature

run ./minion --task=migrations:run --up --group=3-0

Reviewers: shadowhand, rjmackay

Reviewed By: shadowhand, rjmackay

Subscribers: lkamau, srutto

Maniphest Tasks: T192

Differential Revision: https://phabricator.ushahidi.com/D85