Add node 'relocate' command.

This adds a new node command that allows changing the datacenter and rack information in `cassandra-topology.properties` (I need this in order to write a test for [JAVA-315](https://datastax-oss.atlassian.net/browse/JAVA-315)).

Example:

    ccm create twodc -n1:1 -s -b -v 2.1.0-rc1
    ccm node1 relocate dc1 r2