Combine 'node', 'createNodeSet' parameters for ADDREPLICA API

The ADDREPLICA "collection-admin" API has two parameters that are redundant with one another:

• node exists for users to explicitly specify the node to place the new replica on.
• createNodeSet lets users specify a number of nodes that Solr will then choose a "winner" from.

When createNodeSet's value is a single node though, it behaves identically to using the separate node option (AFAIK).

If this is truly the case (and I'm not just missing something), we should deprecate node and instead direct all users to use createNodeSet instead.