Fixed UI bug with broken plugins page

Horizon requires id from returned objects but Savanna plugins
don't have such ones. Actually this code was in savanna client
when it was in savanna-dashboard code:
https://github.com/openstack/savanna-dashboard/blob/0.3a1/savannadashboard/api/plugins.py#L26-L30

Fixes bug#1239676

Change-Id: I6326d0478f5ddbfc4258450e6c0da88cd518de62