This allows update_pending_ranges(), invoked on keyspace creation, to
succeed in the presence of keyspaces with per-table replication
strategy. It will update only vnode-based erms, which is intended
behavior, since only those need pending ranges updated.
This change will also make node operations like bootstrap, repair,
etc. to work (not fail) in the presence of keyspaces with per-table
erms, they will just not be replicated using those algorithms.
Before, these would fail inside get_effective_replication_map(), which
is forbidden for keyspaces with per-table replication.
94e1c7b859