Set remote_groups configuration

In order to use the current version of the ckanext-harvest plugin I have
added the configuration remote_groups to the CKAN group harvester. By
setting it to the value 'only_local' remote groups which are present in
the local CKAN as well are copied.

Signed-off-by: Konrad Reiche <konrad.reiche@fokus.fraunhofer.de>