filters/ccs2needlexml: prefix warning msg with WARNING

IOW, follow the conventions/be consistent.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
filters/ccs2needlexml[cluster]: use base64 for generated key

... so as to make it shorter while carrying the same amount of
information bits

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
run-check: filter corosync's "key" out = make it passing again

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