[CX] Add a mechanism to control bucket config sources in DCP client

Currently, the list of nodes to get bucket configuration from is dynamically grown from the seed node by adding all nodes in the bucket configurations.

We need this list to be controlled differently for different uses.

For example, for an internal bucket, we want the list to be static such that we only request bucket configurations from the NS server on the same host.

For external bucket, do we want the list to only be growing to handle certain failures.. or do we want the list to contain the nodes found in the latest fetched configurations.