Ignore system keyspaces

[scardena]

I've found that when you don't pass a keyspace, this will try to export all keyspaces including "system_schema, system_auth, system" etc, which can't be overwritten, as I get errors like:

system_schema keyspace is not user-modifiable. or Cannot CREATE <keyspace system_auth>

I. am not familiar with cassandra, but even if we find a way to overwrite system keyspaces, it doesn't sound like we should be doing it.
A flag like --ignore-system when used without --keyspaces should be able to copy all keyspaces except the system ones.