data path is reset when node is rebalanced out so 1.8.1behavior will be different when node is added back to the cluster

To reproduce this:-
1) Set data_path of 2 nodes as /mnt
storage path after fresh installation of nodes:-
storage":{"ssd":[],"hdd":[

{"path":"/mnt","quotaMb":"none","state":"ok"}

]}

2) Rebalance in a node
after rebalancing in:-
"storage":{"ssd":[],"hdd":[

{"path":"/mnt","quotaMb":"none","state":"ok"}

]}

3) Rebalance out a node
after rebalancing the node out
"storage":{"ssd":[],"hdd":[

{"path":"/opt/couchbase/var/lib/couchbase/data"}

]]

4) Rebalance in the same node that is removed
after rebalancing the node out
"storage":{"ssd":[],"hdd":[

{"path":"/opt/couchbase/var/lib/couchbase/data"}

]]

The storage path should get updated to /mnt instead of the default path.

This is a regression from 180, cross checked this with 180 build.