Allow users to set index_path post upgrade to 2.0

Use case:-
1) Users upgrade to 20 from 181
2) They decide to separate out partition for indexes. Currently there is no way to configure just index_path without wiping out data_path or reconfiguring the cluster.
 Currently, issuing the data_path command will wipe out the database files.



One can configure the data path
curl -d path=/var/tmp/test http://localhost:8091/nodes/self/controller/settings

This does not seem to work...
curl -d index_path=/data2 http://localhost:8091/nodes/self/controller/settings