Fixes#22688
If we set a dc rf to zero, the options map will still retain a dc=0 entry.
If this dc is decommissioned, any further alters of keyspace will fail,
because the union of new/old options will now contained an unknown keyword.
Change alter ks options processing to simply remove any dc with rf=0 on
alter, and treat this as an implicit dc=0 in nw-topo strategy.
This means we change the reallocate_tablets routine to not rely on
the strategy objects dc mapping, but the full replica topology info
for dc:s to consider for reallocation. Since we verify the input
on attribute processing, the amount of rf/tablets moved should still
be legal.
v2:
* Update docs as well.
v3:
* Simplify dc processing
* Reintroduce options empty check, but do early in ks_prop_defs
* Clean up unit test some
Closesscylladb/scylladb#22693
(cherry picked from commit 342df0b1a8)
(Update: workaround python test objects not having dc info)
Closesscylladb/scylladb#22876
d566010412