Follow-up of "Add support for external plugins"

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
filters/ccs2needlexml: handle secauth on vs. missing keyfile

Also extend corosync.rng (target=needle format) with missing key{,file}.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>