jsonrpc: StorageDomain_attach

Align naming between API.py and schema.

Change-Id: Idb590d892a5d60bf508e22e5bf58a67d9cacb0e1
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35536
Reviewed-by: Adam Litke <alitke@redhat.com>