restapi: Add system permissions resource

Currently there is no resource to manage system permissions directly,
they are managed indirectly using the users and groups resources. This
patch adds a new resource avaiable in /ovirt-engine/api/permissions that
will handle these permissions.

Change-Id: I3119384b3e7036b0d793a71739d77a1ac5a9cf44
Bug-Url: https://bugzilla.redhat.com/1018552
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>