Making setStorageDomainDescription the SPM command

Change-Id: Idca7b7b6642c222d88a6cd2a94d4033c0c3ef70b
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17198
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>