bdr.table_set_replication_sets should permit unsafe commands

bdr.table_set_replication_sets should permit unsafe commands

In 4005c1d bdr_replicate_ddl_command was changed not to override the
command filter, but this broke replication sets in UDR, since they have
to use SECURITY LABEL.

SET bdr.permit_unsafe_ddl_commands on bdr.table_set_replication_sets
since it knows what it's doing.

Fixes #67

Note: The ddl/replication_sets tests cannot be run as-is on UDR because
UDR does not replicate changes to replication sets. We really need
separate tests for this that cover UDR (or BDR+subscribe).