missing validation for cross cluster replication options

There are some combinations which are not currently checked/disallowed:

1. the cluster is managed - externalNodes specified for replicas but nodes are not exposed (we don't complain currently)

2. if sharding is enabled and externalNodes specified for rs0 - we should probably insist that config rs also has external nodes specified (and vice versa)

3. validation for votes and priority connection (Non-voting (i.e. votes is 0) members must have priority of 0.) - this is mongodb rule

4. -if cluster is unmanaged - should we disallow specifying external nodes (currently we don't check)?- for this one decided that it wouldn't be good because when migrating from unmanaged to managed

If I find some others I'll add to this list.