DatanodeAdminManager Improvements

1. Use Collection type Set instead of List for tracking nodes
2. Fix logging statements that are erroneously appending variables instead of using parameters
3. Miscellaneous small improvements

As an example, the node variable is being appended to the string instead of being passed as an argument to the trace method for variable substitution.

LOG.trace("stopDecommission: Node {} in {}, nothing to do." +
          node, node.getAdminState());