nodetool rebuild from DC lets you pass invalid datacenters

If you pass an invalid datacenter to nodetool rebuild, you'll get an error like this:

Unable to find sufficient sources for streaming range (3074457345618258602,-9223372036854775808] in keyspace system_distributed

Unfortunately, this is a rabbit hole of frustration if you are using caps for your DC names and you pass in a lowercase DC name, or you just typo the DC.

Let's do the following:

1. Check the DC name that's passed in against the list of DCs we know about
2. If we don't find it, let's output a reasonable error, and list all the DCs someone could put in.
3. Ideally we indicate which keyspaces are set to replicate to this DC and which aren't