KCC: Help RW DCs to ignore RODCs when doing kcc

KCC: Help RW DCs to ignore RODCs when doing kcc

As far as writable DCs are concerned, RODCs don't even exist. So we make
tables that leave out the RO ones.

An RODC needs to know itself as well as writable DCs, so we add it in
that case.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>