api: domainDict is optional in connectStoragePool

This patch fixes a mismatch between the schema and the actual
connectStoragePool implementation.

Change-Id: I05b4f5073a33b2479404298ddafd4802e949e8f4
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24089
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>