Bug 3166 ui-backend introduced

Bug 3166 ui-backend introduced

This is a backend based on RESTCONF.
Purpose of the backend is to provide data which has to be calculated by difficult processing
i.e. it reuses policy resolution process from groupbasedpolicy module.
UI can call RPCs on this backend.

Backend provides these RPCs:
 - get-endpoints-from-endpoint-group: Returns endpoints which are
        in the given endpoint group.
 - get-subjects-between-endpoint-groups: Returns subjects between
        endpoint group pairs where endpoint groups are from the given tenant.

Change-Id: I8490a7217c5725ccdfc6dda3467d3ba91c76eeb9
Signed-off-by: Martin Sunal <msunal@cisco.com>