core: wrong param name for fencing policy

In the api schema and API.py the fencing policy param is named policy
whereas engine passes fencingPolicy so value is not properly mapped.
This patch fixes the naming.

Change-Id: I291eb7973afa023e2f3d49bd09b966ce6ae9e333
Bug-Url: https://bugzilla.redhat.com/1143860
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Signed-off-by: Martin Perina <mperina@redhat.com>