Confusing disk usage stats on Admin console

Disk usage stats from node:

```
[root@localhost data]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/VolGroup-lv_root
3.7G 3.0G 509M 86% /
tmpfs 2.9G 227M 2.7G 8% /dev/shm
/dev/sdb1 30G 881M 28G 4% /opt/couchbase/var/lib/couchbase/data
/dev/sdc1 9.9G 4.0G 5.4G 43% /opt/couchbase/var/lib/couchbase/index
/dev/sda1 485M 124M 336M 27% /boot
```

Disk usage stats from REST API:

```
[asingh:~/go/src/] $ curl http://ops:password@10.4.2.106:8091/nodes/ns_1%4010.4.2.106 -s | jq ".storageTotals.hdd"

{ "free": 36462821131, "usedByData": 4842504346, "used": 5812900085, "quotaTotal": 42275721216, "total": 42275721216 }

```

From the screenshot attached from admin console disk usage shows 4.5GB(in use) for `/opt/couchbase/var/lib/couchbase/data`, which is incorrect. It's disk usage stats for *both index and data dir* combined.

We should either refer to disk usage stats for data and index dir separately(this will be more useful) or refer it as overall usage by data & index dir.